h(t) = -16t^2 + 64
When the object hits the ground, the height above ground is clearly zero. Since the height above ground is zero, h(t) = 0.
We have:
h(t) = 0
-16t^2 + 64 = 0
16t^2 = 64
t^2 = 64 /16
t^2 = 4
t = sqrt(4)
t = 2
Hence, the answer is 2 seconds.