done

pom.xml

                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-compiler-plugin</artifactId>
                 <configuration>
-                    <source>1.7</source>
-                    <target>1.7</target>
+                    <source>1.8</source>
+                    <target>1.8</target>
                 </configuration>
             </plugin>
         </plugins>
             <artifactId>junit</artifactId>
             <version>4.0</version>
         </dependency>
+        <!-- https://mvnrepository.com/artifact/com.zenjava/javafx-maven-plugin -->
+        <dependency>
+            <groupId>com.zenjava</groupId>
+            <artifactId>javafx-maven-plugin</artifactId>
+            <version>8.8.3</version>
+        </dependency>
+
     </dependencies>
 
 

src/main/java/rocks/zipcode/calcskin/CalcEngine.java

     }
 
     public double add(double v, double v1) {
-        return Double.NaN;
+        return v + v1;
     }
 
     public double subtract(double v, double v1) {
-        return Double.NaN;
+        return v-v1;
     }
 
     public double multiply(double v, double v1) {
-        return Double.NaN;
+        return v * v1;
     }
 
     public double divide(double v, double v1) {
-        return Double.NaN;
+        return v/v1;
     }
+    public double square ( double v){
+        return Math.pow(v , 2.0);
+    }
+    public double root(double v){return Math.sqrt(v); }
+
+    public double expo ( double v , double v1){
+        return Math.pow(v,v1);
+    }
+
+
+
+
 }

src/main/java/rocks/zipcode/calcskin/CalcSkin.java

 import javafx.scene.layout.*;
 import javafx.stage.Stage;
 import javafx.stage.StageStyle;
-import rocks.zipcode.calcskin.CalcEngine;
 
 import java.util.HashMap;
 import java.util.Map;
         launch(args);
     }
     private static final String[][] template = {
-            { "7", "8", "9", "/" },
-            { "4", "5", "6", "*" },
-            { "1", "2", "3", "-" },
+            { "7", "8", "9", "/", "x^2" },
+            { "4", "5", "6", "*",  "rt" },
+            { "1", "2", "3", "-",  "x^y" },
             { "0", "c", "=", "+" }
     };
 
     private DoubleProperty currentValue = new SimpleDoubleProperty();
     private CalcEngine calcEngine = new CalcEngine();
 
-    private enum Op { NOOP, ADD, SUBTRACT, MULTIPLY, DIVIDE }
+    private enum Op { NOOP, ADD, SUBTRACT, MULTIPLY, DIVIDE, SQUARE, ROOT, EXPO }
 
     private Op curOp   = Op.NOOP;
     private Op stackOp = Op.NOOP;
     private VBox createLayout(TextField screen, TilePane buttons) {
         final VBox layout = new VBox(20);
         layout.setAlignment(Pos.CENTER);
-        layout.setStyle("-fx-background-color: silver; -fx-padding: 20; -fx-font-size: 20;");
+        layout.setStyle("-fx-background-color: green; -fx-padding: 20; -fx-font-size: 20;");
         layout.getChildren().setAll(screen, buttons);
         handleAccelerators(layout);
         screen.prefWidthProperty().bind(buttons.widthProperty());
 
     private TextField createScreen() {
         final TextField screen = new TextField();
-        screen.setStyle("-fx-background-color: aquamarine;");
+        screen.setStyle("-fx-background-color: black;");
         screen.setAlignment(Pos.CENTER_RIGHT);
         screen.setEditable(false);
         screen.textProperty().bind(Bindings.format("%.0f", currentValue));
             case "-": triggerOp.set(Op.SUBTRACT); break;
             case "*": triggerOp.set(Op.MULTIPLY); break;
             case "/": triggerOp.set(Op.DIVIDE);   break;
+            case "x^2": triggerOp.set(Op.SQUARE);   break;
+            case "rt": triggerOp.set(Op.ROOT);   break;
+            case "x^y": triggerOp.set(Op.EXPO);   break;
         }
         return triggerOp;
     }
 
     private void makeOperandButton(Button button, final ObjectProperty<Op> triggerOp) {
-        button.setStyle("-fx-base: lightgray;");
+        button.setStyle("-fx-base: lightblue;");
         button.setOnAction(new EventHandler<ActionEvent>() {
             @Override
             public void handle(ActionEvent actionEvent) {
 
     private Button makeStandardButton(String s) {
         Button button = new Button(s);
-        button.setStyle("-fx-base: beige;");
+        button.setStyle("-fx-base: pink;");
         accelerators.put(s, button);
         button.setMaxSize(Double.MAX_VALUE, Double.MAX_VALUE);
         return button;
     }
 
     private void makeClearButton(Button button) {
-        button.setStyle("-fx-base: mistyrose;");
+        button.setStyle("-fx-base: orange;");
         button.setOnAction(new EventHandler<ActionEvent>() {
             @Override
             public void handle(ActionEvent actionEvent) {
     }
 
     private void makeEqualsButton(Button button) {
-        button.setStyle("-fx-base: ghostwhite;");
+        button.setStyle("-fx-base: violet;");
         button.setOnAction(new EventHandler<ActionEvent>() {
             @Override
             public void handle(ActionEvent actionEvent) {
                     case SUBTRACT: currentValue.set(calcEngine.subtract(previousValue.get(), currentValue.get())); break;
                     case MULTIPLY: currentValue.set(calcEngine.multiply(previousValue.get(), currentValue.get())); break;
                     case DIVIDE:   currentValue.set(calcEngine.divide(previousValue.get(), currentValue.get())); break;
+                    case ROOT:   currentValue.set(calcEngine.root( currentValue.get())); break;
+                    case SQUARE:   currentValue.set(calcEngine.square( currentValue.get())); break;
+                    case EXPO:   currentValue.set(calcEngine.expo(previousValue.get(), currentValue.get())); break;
                 }
             }
         });