Question 631027
Given:
s(t)=-16t^2+48t 
.
S(t) is zero when it hits the ground -- so, set s(t) to zero and solve for t:
0 = -16t^2+48t 
0 = 16t^2-48t 
0 = t^2-3t 
0 = t(t-3)
.
t = {0, 3}
throw out the 0 (beginning of throw) leaving:
t = 3 seconds