The car has an average acceleration of
[tex]a_{\rm ave} = \dfrac{\Delta v}{\Delta t} = \dfrac{0\frac{\rm m}{\rm s} - 20\frac{\rm m}{\rm s}}{4\,\mathrm s} = -5\dfrac{\rm m}{\mathrm s^2}[/tex]
Then, by Newton's second law, the average force exerted by the brakes on the car is
[tex]F_{\rm ave} = ma_{\rm ave} = (1200\,\mathrm{kg})\left(-5\dfrac{\rm m}{\mathrm s^2}\right) = \boxed{-6000 N}[/tex]
which is to say, the average force has a magnitude of 6000 N and is directed opposite the car's motion.
