I think you said everything but the answer.
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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
This is a problem of accelerated motion, where the acceleration involved is the gravitational acceleration:
Therefore, we can use the following formula to solve the problem:
where