A) If the box moves at a constant velocity, what we know about the forces acting on the object is; the sum of the forces acting on the object must be zero.
B) If the box moves at a constant velocity, the force that opposes the motion of the box is; Frictional Force of -20 N
C) If the box experiences a force of 15 N to the left, along with the 20 N force acting to the right, the net force on the box is; F_net = 5 N
We are told that a box is being pulled across a horizontal surface by a 20N force to the right.
In this case the opposing force will have to be the frictional force which will be -20 N to balance out the applied force.
Thus, the net force will be;
F_net = 20 - 15
F_net = 5 N
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