For similar triangles, the side lengths are proportional. so, you can write a proportion relating the two pairs of vertical and horizontal side lengths. for the line y = 2x, a triangle can be drawn 2 units up and 1 unit right from the origin. write a proportion relating the side lengths of the triangle to any other triangle that has the line as a hypotenuse, a horizontal length of x units, and a vertical length of y units.