The zeros are the values of t for which f(t) = 0.
i.e. -16t^2 + 96t = 0
16t^2 - 96t = 16t(t - 6)
16t = 0 or t - 6 = 0
t = 0 or t = 6
Therefore, the zeros are 0, 6
The time taken for the ball to hit the ground is the value of t when f(t) = 0.
i.e. t = 6.