Answer: Air resistance
Explanation: If you subtract the net force from the weight of a falling object, you would be solving for a force that is not being balanced from the other forces.
The net force (with no external forces) on the falling object would just be the force of gravity. So, any force that unbalances the force of gravity would be what you are solving for. This can be air resistance, friction (if the object is sliding against a wall), or applied forces (pushing the object while it falls).