Notice that the path traveled by the rocket will model that of a parabola. Also notice that when the rocket reaches the ground, the height will be 0.
So, we set our expression for Height equal to 0:
Now rearrange so that it is in quadratic form:
Now solve for t. Remember that you will get two values, since it is a quadratic. One of these roots will be 0, because at time 0, the rocket is still on the ground and therefore has a height of 0.
The second root will tell you the total time that the rocket was in the air.