Question

if a toy rocket is launced vertically upward from ground level with an inital velocity of 128 feet per second, then its high h, after t seconds is given by the equation h(t)=-9t^2 +81 round to the nearest foot when does the ball hit the ground?

Answer 1

The toy rocket hits the ground when h(t) = 0.
So, h(t) = 0 = -9t^2 + 81.

Solving the equation,
-9t^2 = -81
Dividing by -9
t^2 = 9
Taking the square root.
t = 3

The answer is 3 seconds.