Distance moved in 3 sec $ = \frac{9}{2}g$
Remaining distance $ = \frac{{16}}{2}g$
If $t$ is the time taken by the stone to reach the ground for the remaining distance then
$ \Rightarrow \frac{{16}}{2}g = \frac{1}{2}g{t^2} \Rightarrow t = 4\;\sec $