To determine the net force acting upon an object we need to do the vector addition of the forces acting on it.
In this case we have two forces, one acting to the left and one to the right. We need to establish which of this ways is the positive direction. Let's assume the right direction is the positive direction; for this convention the 20 N forces is positve and the 10 N is positive; then the net force would be:
Since the resulting net force is positive this means, for our convention, that the net force points in the right direction. Therefore, the net force is 10 N to the right.