Question 211479
The problem gives you:
h=-16t^2+64t+80
where
h is height in feet
t is time in secs
.
If it hits the ground, h = 0 so 
set h=0 and solve for t:
h=-16t^2+64t+80
0=-16t^2+64t+80
0=-4t^2+16t+5
Solving using the quadratic equation yields:
t = {-0.29, 4.29}
We can throw out the negative solution leaving
t = 4.29 seconds
.
Details of quadratic to follow:
*[invoke quadratic "t", -4, 16, 5 ]