h(t) = -16t2 + 64
Time to reach ground:
h(t) = 0
-16t2 + 64 = 0
Solve for t, keeping only the positive solution.
Time to fall 36 ft:
Since it is dropped from 64 ft, its height after falling 36 feet is 64-36=28 ft.
28 = -16t2 + 64
-16t2 + 36 = 0