If the arrows are in the same direction then you have to add them. If the arrows are in opposite you have to take away. For example for question 5 8-4=4 so thE net force will be 4 and the arrow will be left as 8N is bigger. For question 4 they are in the same direction so you add them it will be 9N to the right. For question 3 they are in opposite so it will be 0 and both arrows are equal so the net force is 0 and then object is stationary
Hope this helps
