A rock is thrown upward with a velocity of 25 meters per second from the top of a 50 meter high cliff, and it misses the cliff on the way back down. When will the rock be 7 meters from the water, below?

Answer 1

your height equation must be

h = -4.5t^2 + 25t + 50

so you want to solve:
7 = -4.5t^2 + 25t + 50
4.5t^2 - 25t - 43 = 0
use the quadratic formula, discard any negative value of t