b. Both forces act in the same direction, so add their magnitudes.
Net force: 2 N + 4 N = 6 N to the right
Change in motion: the box will move to the right.
c. The forces act in opposite directions, so subtract the magnitudes. The motion is in the direction of the force with greater magnitude.
Net force: 10 N - 8 N = 2 N to the right
Change in motion: the box will move to the right.