Answer
$$t=3$$
Work Step by Step
We are given the following equation:
$$ h=48t-16t^2$$
Thus, plugging in $0$ for $h$ and looking for the solution besides t=0, we find:
$$ 0=48t-16t^2 \\ t_{1,\:2}=\frac{-48\pm \sqrt{48^2-4\left(-16\right)0}}{2\left(-16\right)} \\ t=3$$