Looking at the normal force and the gravitational force, they are both of equal magnitude (3N), and are in opposite directions ( up and down). This means they cancel out - their overall force is 0.
Moving on to the sideways forces, we only have friction (5N). There is no force to the right to cancel this out, so the overall force must be 5N left.
Hope this helps! Let me know if you have any questions :)