Question 1034820
For h(t) = -16t^2 + 52t + 48, which is the height of the rock as a function of time, we want to know at what time the height equals zero.  So we set it equal to zero and solve for t...
-16t^2 + 52t + 48 = 0  (now divide by -4)
4t^2 - 13t - 12 = 0
which is factorable into
(4t + 3)(t - 4) = 0
so that
t = 4 seconds