To solve this problem it is necessary to apply the equations given in Newton's second law as well as Hooke's Law.
Since Newton's second law we have that force is
F = mg
Where,
m = mass
g = gravity
From the hook law, let us know that
F = -kx
Where
k = Spring constant
x = Displacement
Re-arrange to find k,

PART A ) We can replace the Newton definitions here, then

Replacing with our values we have that


Therefore the required value of the spring constant is 
PART B) We can also equating both equation to find the mass, then


Replacing with tour values we have


Therefore the mass of the player can be of 76.8Kg, then the player is eligible to play because the mass is less than 85Kg