Answer:
t = 8 seconds
Step-by-step explanation:
Since this is a quadratic equation, there will be two values for which H = 0, (the ball is on the ground), so we'll just solve the equation and the first value will be the moment when the cannonball is launched, and the other will be when the cannonball falls to the ground.
0 = -5t² + 40t
Factorizing, since the two terms are being multiplied by t:
t(40 - 5t) = 0
the equation is true whenever t = 0 or t = 8, so we know that at the zeroth second, the ball was being launched, and at the 8th second, the ball was on the ground.
Here is how the graph looks like:
