Right... you need to see that there are two congruent triangles. They share a common angle at the top vertex and they have both a right angle. So, they are congruent.

So, next, you take the two longest sides of the triangles, the hypotenuses. The shorter hypotenuse over the longer hypotenuse should be equal to a shorter side over a longer side of two corresponding sides of both triangles.

For this, I will take the side a. This will correspond to the last dies of the larger triangle. And this side is given by Pythagoras' Theorem. Let's call this side x.

x is given by h^2 = 4^2 + x^2, so x =

Substitute that in your above equation and you are done after some manipulation