Question 1146590
Hits ground when H=0
so -16t^2-12t+160=0
divide by 4
-4t^2-3t+40=0 
4t^2+36-40=0
t=(1/8)(-3 +/- sqrt (9+640); sqrt 649=25.48
positive root only t=22.48/8=2.81 sec

{{{graph(300,300,-3,5,-50,200,-16x^2-12x+160)}}}