Question

A rock is thrown down from a cliff that is 180 m high. The function

h(t) = -5t^2 – 10t + 180 gives the approximate height of the rock
above the water, where h(t) is the height in metres and t is the time
in seconds. When will the rock reach a ledge that is 105 m above
the water?

Answer 1

t = 3

Explanation:

105 = -5t^2 -10t + 180

-75 = -5t^2 -10t

15 = t^2 +2t

t^2 +2t -15 = 0

(t+5)(t-3)

t = -5, 3

There's no such thing as negative time so you use the positive answer