a) The Maple Leaf is pushed by a force of 50N by the Canuck, hence, by Newton's Third Law, the Canuck receives 50N of force from the Maple Leaf.
b) Use F = ma for this one:
First, for the Maple Leaf,
For the Canuck, the problem is slightly different. He experiences 50N of force in the opposite direction to his velocity. This means we need to make the force negative, and he will deccelerate.
c) You can try tackling this with intuition. If a car crashes into a stationary car, it will feel less force than if two cars collide head on at speed.
The Maple Leaf feels 50N from the Canuck, but because he also pushes with 50N, by Newton's Third Law, he feels an additional reaction force of 50N, hence he feels a total force of 100N. This is the same for the Canuck.
In this problem, the acceleration of both players is negative, since the force acts in the opposite direction to their velocity. You should be able to work out the new accelerations.