a 1200 kg car goes from 15m/s in 16s. What is the average force acting on it?
From 15 m/s to what higher speed does it move in that interval? Or do you mean it goes from zero to 15 m/s? You have not copied the full question.
You need to compute the acceleration, a, and use the equation
F = m a
F will be the force in Newtons, if mass m and a are in the proper units.