To find the net force acting on the runner, you can subtract the air resistance force from the running force since they are acting in opposite directions.
The running force is 325 N to the right, and the air resistance force is 45 N to the left.
Net force = Running force - Air resistance force
Net force = 325 N - 45 N = 280 N
So, the net force of the runner is 280 N.