Solve the all parts of given problem:

Problem: A 55-kg soccer player jumps vertically upwards and heads the 0.45-kg ball as it is descending vertically with a speed of 26 m/s.

Part A: If the player was moving upward with a speed of 4.0 m/s just before impact, what will be the speed of the ball immediately after the collision if the ball rebounds vertically upwards and the collision is elastic? m/s

Part B: If the ball is in contact with the player's head for 15 ms, what is the average acceleration of the ball? (Note that the force of gravity may be ignored during the brief collision time.) m/s2

Please explain each step and equations used for above problem.