double sceneWidth = 400.0, sceneHeight = 300.0;
...

Polygon triangle = new Polygon( 0, sceneHeight, sceneWidth, sceneHeight, sceneWidth/2, 0);

Better Triangle

Here is another program that draws a triangle using the JavaFX class Polygon . This time, all vertices are calculated using the scene dimensions.

import javafx.application.*;
import javafx.stage.*;
import javafx.scene.Scene;  
import javafx.scene.shape.*;
import javafx.scene.paint.*;
import javafx.scene.layout.*; 

public class TriangleDemo extends Application
{
  public void start( Stage primaryStage )
  {
    double sceneWidth = 400.0, sceneHeight = 300.0;
    double sceneCenterX = sceneWidth/2; 
    double sceneCenterY = sceneHeight/2; 
    
    // height and width of the triangle
    double triHeight = sceneHeight*0.75;
    double triWidth  = sceneWidth*0.5;
    
    // y coordinate of base of triangle
    double triBaseY = sceneCenterY+triHeight/2;
    
    // top vertex of the triangle
    double triTopX = sceneCenterX;       
    double triTopY = triBaseY-triHeight;  
    
    // left and right vertices of the triangle
    double triLeftX  = sceneCenterX-triWidth/2;
    double triLeftY  = triBaseY;
    double triRightX = sceneCenterX+triWidth/2;
    double triRightY = triBaseY;
 
    // array of vertices x,y, x,y, x,y
    // in clockwise order
    double[] vertices = { triRightX, triRightY, triTopX, triTopY, triLeftX, triLeftY };
    Polygon triangle = new Polygon( vertices );
    triangle.setFill( new Color( 0.8, 0.2, 0.1, 1.0) );
 
    // Axes through center of scene
    Line lineH = new Line( 0, sceneCenterY, sceneWidth, sceneCenterY ) ;
    lineH.setStroke( new Color( 0.2, 0.2, 0.2, 0.5)  );
    Line lineV = new Line( sceneCenterX, 0, sceneCenterX, sceneHeight ) ;
    lineV.setStroke( new Color( 0.2, 0.2, 0.2, 0.5)  );
    
    Pane pane = new Pane( triangle, lineH, lineV );
    Scene scene = new Scene( pane, sceneWidth, sceneHeight, Color.FLORALWHITE );
    
    primaryStage.setTitle("Triangle Demo");
    primaryStage.setScene( scene );
    primaryStage.show();
  }
}

The constructor for Polygon looks like this:

public Polygon(double... points) // vertices as two X Y values, in clockwise order.

It is often more convenient to calculate the vertices and put them into an array of double arranged as pairs X-Y in clockwise order. Then the array can be used in the constructor.

// array of vertices x,y, x,y, x,y in clockwise order
double[] vertices = { triRightX, triRightY, triTopX, triTopY, triLeftX, triLeftY };
Polygon triangle = new Polygon( vertices );