To find the net force of 15 N and -10 N, you simply add the two forces together. The positive sign indicates a force in one direction, while the negative sign indicates a force in the opposite direction.
Net force = 15 N + (-10 N)
Net force = 15 N - 10 N
Net force = 5 N
The net force is 5 N in the direction of the 15 N force.