Help please i cant figure this out

Question

The height of a projectile launched upward at a speed of 20 meters/second from a height of 225 meters is given by the function h(t)=-5sup(t,2)+20t+225. How long will it take the projectile to hit the ground?

Answer 1

9 seconds to hit the ground

divide by -5 to get

t^2 -4t -45 = 0

factor to get

(t-9)(t+5) = 0 ignore the negative solution

t-9=0

t = 9 seconds