When the rick hits the ground, the height would be 0. So plug zero in for h and then solve.
0 = 78.4t – 4.9t²
This can be solved by the quadratic equation, it also factors.
0 = 4.9t(16-t)
Setting the factors equal to zero and solving
4.9t = 0 and 16-t=0
t=0 and t=16
t=0 is when the rock, LEAVES the ground. The time it hits the ground would be 16 seconds.