if the hypotenuse of the 10unit high triangle is 12 (because the two sides of the parallelogram are equal), then you can determine the other length. if you multiply 12 by 1.5 you get 18, so you'd multiply all other sides of the triangle to make it to scale. 12 x 1.5 = 18, and 10 x 1.5 = 15. h = 15
