algebra
posted by Paula on .
A rock is thrown upward with a velocity of 15 meters per second from the top of a 26 meter high cliff ad i misses the cliff on the way back down. when will the rock be 5 meters from the water below?

height = 4.9t^2 + 15t + 26
When the rock is 5 metres above the water,
5 = 4.9t^2 + 15t + 26
4.9t^2  15t  21 = 0
t = (15 ± √636.6)/9.8
= 4.1052 seconds or a negative
After appr 4.1 seconds, the rock is 5 metres above the water.