Explain why action-reaction forces do not cancel each other out, resulting in zero net force.

Action and reaction are acting on two different objects. A net force can only be calculated from forces acting on a single object or system. When you have exactly two equal and opposite forces acting on the same system, the net force is zero, and the system will not accelerate. But the two forces implied by Newton's 3rd Law are acting on different things. 

W= fd
where f is an applied force and d is the distance over which it is applied. when a parachuting skydiver is at constant velocity, he is experiencing "terminal velocity", the point at which his acceleration down is canceled out by air resistance. that means that the resisting force of the air is equal to his weight, but in the opposite direction. with this, we can solve for the work done by the force.
w = fd
w = (92.0)(9.81 m/s^2) (325)
w= 2.93 x 10^5 j
the value for work in this case is positive because it is pushing up on the skydiver, who has a downward force of gravity.
therefore the answer is 2.93 x 10^5 j