The net force on the car is equal to 661.5 N.
Why?
We need to calculate the net force on the car and not the exerted by the trailer on the car. However, if we were asked to calculate the force exerted on the car by the trailer, we must remember that ar is pulling (and moving) the trailer, the net force exerted by the car on the trailer will be the same (with opposite direction) that the force exerted by the trailer on the car.
So, calculating we have:

Hence, we have that the net force on the car is equal to 661.5N.
Have a nice day!