You have to use the Pythagoras theorem: c² = a²+b²
You have been given the c side, the diagonal side, and the b side the height side. You need to square both of them, take them away from each other and then get the square root of that answer;
120²-64² = 10302
√10302 = 101.49
The answer is 101.49ft,
Hope this helps! :)