Answer
given,
mass of soccer player = M = 65 Kg
mass of football = m = 0.45 Kg
speed of the ball which is descending (u₂)= 23 m/s
Speed of the player vertically upward(u₁) = 4 m/s
a) Using conservation of momentum
M u₁ + m u₂ = M v₁ + m v₂
65 x 4 - 0.45 x 23 = 65 v₁ + 0.45 v₂
65 v₁ + 0.45 v₂ = 249.65................(1)
we know,

e = 1 for elastic collision

.....................(2)
now putting value of v₂ in equation(1)
65 v₁ + 0.45 (27 + v₁) = 249.65
65.45 v₁ = 237.5
v₁ = 3.63 m/s
v₂ = 30.63 m/s
b)
acceleration of ball =

acceleration of ball =

acceleration of ball =2437.73 m/s²