3. A soccer ball is thrown from the top of a cliff. Its height, h, in meters, above the sea after t seconds can be modelled by h = -5t^2 + 21t + 120. How long will the ball take to fall 20 m below its initial height?

Answer 1

The equation for h(t) tells you that the initial height (when t=0) is 120 m.
just solve in the usual way:
-5t^2 + 21t + 120 = 120-20