Answer:
See Explanation
Step-by-step explanation:
According to the diagram, the +3.5 nC arrow is in the positive direction whereas the +4 nC arrow is in the negative direction. Thus, the question defines the positive direction as the "right" and the negative direction as the "left." Since the instructions ask you to calculate the "force exerted on the +4 nC charge," the answer would be a negative force, or rather a force acting to the left.