The question is: After a football is punted its height, $\displaystyle h$, in meters above the ground at $\displaystyle t$ seconds can be modelled by $\displaystyle h(t)=-4.9t^2+21t +0.45$. Determine the restricted domain of this model.

I was also asked to find the maximum height and what time it reached that height and had no difficulty.

I have no idea at all how to find the restricted domain.