The net force is defined as the vector sum of all the forces that are present in an object. The formula for net force is F=ma. Everything else cancels out like the force of gravity on the car, and the reactive force of the road surface, etc. It can be concluded that the force exerted by the trailer on the car is the same as that exerted by the car on the trailer. Given that the acceleration is 1.75 m/s^2, the car is 1200 kilograms, and the trailer is 540 kilograms. Thus, the net force acting on the car is equal to 21qkm/hr^2