The expression - 5t^2 + 40t predicts the height, in meters, of an object t seconds after a person launches it into the air. Using this expression, how many seconds will it take the object to hit the ground?

Question

How do I solve this?

Answer 1

when it hits the ground, the height is zero, right?
So, just solve
-5t^2 + 40t = 0
-5t(t-8) = 0
...