To get started note that the height from the pendulum to the pivot is 48 cos(x), and that equals 48-8 = 40. So cos(x) = 40/48 = 5/6.

Next - for angle 2x the height from the pendulum to the pivot is 48 cos(2x). Use the cos(2x) identity and find what that is. Then subtract that from 40 to find the difference.