Imagine this as a triangle. You have the rectangle (window) and you have one side which is 5 inches, then its 10 inches across so thats the hypotenuse.
a^2+b^2=c^2
so if we apply a = 5 and c=10
c^2 - b^2 = a^2
I got 8.6 but that doesn't seem right, Im stuck on this too