F.4 free fall

1.A rock is released from the second floor and it strikes the ground at a speed of v.
If it is released from the seventh floor, what is the speed ( in terms of v) when it strikes the ground

2.A spacecraft falling with a speed of 200m/s prepares to land on the Moon.
By firing its retro-rockets, the spacecraft can decelerate uniformly at 20m/s/s.
The spacecraft will land safely at 5m/s or below.w
When to fire the retro-rockets will gives a safe landIng?

可以可以解釋埋呀 唔該曬!