How long does it take for the ball to hit the ground at -5t^2 + 10t + 320

if t is time, then we are solving for the value of t that makes the expression equals 0 (height of the ground)

0=-5t^2+10t+320

factor out -5

0=-5(t^2-2t-64)

use quadratic formula

for at^2+bt+c=0

in our case, a=1, b=-2, c=-64

since and we can't have negative time,

which is about 9.062 seconds

17.08

correct me if i am wrong.