Answer
$a). 2.29 s$
$b). 16.5 m/s$
Work Step by Step
$v^{2}=u^{2}+2aS$
$0=6^{2}-2\times 9.8\times S$
$S=1.84m$
So, the stone goes upto a height of 1.84 m.
So, the stone has to travel =$1.84+12=13.84m$ before it hits the ground.
Now, $v=u+at$
or, $0=6-9.8t$
i.e. $t=0.61s$
$S=ut+\frac{1}{2}at^{2}$
$13.84=0+0.5\times 9.8t^{2}$
$t=1.68s$
a). So, time of total flight $=0.61s+1.68s=2.29s$
b). Also, velocity just before hitting the ground$=u+at=0+9.8\times1.68=16.5m/s$
