We have 5 newtons of force pushing to the right, and 3 newtons of force pushing to the left. We can place the positive direction on the right hand side and the negative direction on the left.
This means we can think of "3 newtons of force to the left" as the integer -3
That way we can then add 5 to -3 getting 5+(-3) = 2. The result is positive, meaning the net force is pushing to the right. Specifically, 2 newtons of force are pushing to the right.
It's like saying there are 5 people pushing to the right and 3 pushing to the left. From each side, 3 people will pair up and they'll cancel each other out. The 2 extra people pushing right will ultimately win out.
