When calculating net force, we subtract opposing forces, and add together forces going in the same direction! With opposing forces, since you can’t have a negative net force, the smallest force will be subtracted from the largest force!
In the top left corner, an arrow going right is 4N, and an arrow pushing left is also 4N. Because they are equal numbers and in opposite directions, they cancel each other out, for a balanced net force of 0N.
In the bottom left, we have 4N pointed right and 5N pointed left. 5N (largest value) - 4N (smallest value) gives you an unbalanced net force of 1N.
On the top right both forces, 6N and 3N, are pointed in the same direction. Because of this, we add them to get a total 9N net force.
On the bottom right, 7N are going left and 2N are going right. Because they are opposing forces, we subtract 2N from the 7N to get a total of 5N.
