$x=100t-\large\frac{2s}{2}$$t^2$
The velocity with which the missile strikes the ground when the missile strikes the ground $x=0$
Therefore $100t -\large\frac{25}{2}t^2=0$ or
$\large\frac{t}{2}$$(200-25t)=0=>t=0,8$
Therefore the missile reaches the ground again at $t=8 \;secs$
Substitute $t=8$ in (ii) is obtain the velocity with which it strikes the ground.
The velocity $=100-25 \times 8$
$\qquad\quad=-100 \;m/sec$
(The negative sigh indicates the downward direction)