Answer
The average frictional force is $8400~N$ and this force is exerted in the opposite direction of the car's motion.
Work Step by Step
We can find the acceleration:
$v_f = v_0+at$
$a = \frac{v_f-v_0}{t}$
$a = \frac{0-28~m/s}{4.0~s}$
$a = -7.0~m/s^2$
The magnitude of acceleration is $7.0~m/s^2$
We can find the average frictional force:
$F = ma$
$F = (1200~kg)(7.0~m/s^2)$
$F = 8400~N$
The average frictional force is $8400~N$ and this force is exerted in the opposite direction of the car's motion.
