public Vector3f calculateD() {
        float x = b.x + c.x - a.x;
        float y = b.y + c.y - a.y;
        float z = b.z + c.z - a.z;
        return new Vector3f(x, y, z);
    }
Returns the coordinates of the 4th corner, calculated by the formula
D = (B + C) - A .

@return the corner location (a new Vector3f)
jme3-core/src/main/java/com/jme3/math/Rectangle.java