To determine the mass of a car, a student (with a friend at the wheel) pushes the car holding a bathroom scale between himself and the car and

To determine the mass of a car, a student (with a friend at the wheel) pushes the car holding a bathroom scale between himself and the car and carefully maintains a constant reading of 400 N on the scale while the car accelerates on level ground. At the conclusion of the experiment his friend reports that the car accelerated from rest to 14.0 km/hr in 12.0 s. What was the mass of the car?