Hello
This is a problem of accelerated motion, where the acceleration involved is the gravitational acceleration:

, and where the negative sign means it points downwards, against the direction of the motion.
Therefore, we can use the following formula to solve the problem:

where

is the initial vertical velocity of the athlete,

is the vertical velocity of the athlete at the maximum height (and

at maximum height of an accelerated motion) and S is the distance covered between the initial and final moment (i.e., it is the maximum height). Re-arranging the equation, we get
Elastic Potential Energy because the elasticity in the string stores up the energy.
Force is all the same, but pressure will be higher on 3cm
They had to use thrust because there is no air on the moon meaning that parachutes would be totally useless because parachutes require air to create drag. Fun fact since there is no air on the lunar surface sound waves cannot be made since there are no air molecules for the waves to pass through, so that's why they use radio's in space.