I have done two parts but no idea how to the third one.

Question

The height, h, in meters, of a firecracker fired into the air can be modeled using the function h(t)=-5(x-4)^2+85, where t is the time, in seconds, after the firecracker is fired.
c) Use the graph to estimate
i) how long the firecracker is in the air
ii) the maximum height reached by the firecracker, and how long it takes to reach this maximum height

Answer 1

the time in the air is t when h=0 again. So, just solve for h(t) = 0.

the max height is the vertex of the parabola, clearly when t=4.