BuildEquilateral = function (x, y, length)
{
    var height = length * (Math.sqrt(3) / 2);

    var x1 = x;
    var y1 = y;

    var x2 = x + (length / 2);
    var y2 = y + height;

    var x3 = x - (length / 2);
    var y3 = y + height;

    return new Triangle(x1, y1, x2, y2, x3, y3);
}
Builds an equilateral triangle. In the equilateral triangle, all the sides are the same length (congruent) and all the angles are the same size (congruent).
The x/y specifies the top-middle of the triangle (x1/y1) and length is the length of each side.

@function Phaser.Geom.Triangle.BuildEquilateral
@since 3.0.0

@param {number} x - x coordinate of the top point of the triangle.
@param {number} y - y coordinate of the top point of the triangle.
@param {number} length - Length of each side of the triangle.

@return {Phaser.Geom.Triangle} The Triangle object of the given size.