Solving this Gravity Problem using the Quadratic Formula

Question

A rock is thrown upward with a velocity of 15 meters per second from the top of a 43 meter high cliff, and it misses the cliff on the way back down. When will the rock be 5 meters from the water, below?

Answer 1

h(t) = -4.9t^2 + 15t + 43 = 5

4.9t^2 -15t- 38 = 0

t = 30/9.8 +/-(1/9.8)sqr(15^2 -4(4.9)(-38)) seconds

when the rock is 5 meters above the water

t = 15/4.9 +/-sqr(225 +744.8)

t = (15 +sqr(969.8))/4.9 = about 6 1/4 seconds