Because the car completely stops you can apply the standard formula connecting the acceleration, distance traveled, initial and final speed. Let "a" be the acceleration, v_{i
}= 30m/s (initial speed), v_{f} = 0 -final speed, F -force of friction, "m" - mass. If the distnace traveled is "d", then we have
2ad = v_{f}^{2} - v_{i}^{2} = -900 m^{2}/s^{2}
The force of friction is acting in opposite direction. Thus,
a = F/m = - 6000 N/1200 kg = - 5m/s^{2}
We obtain for d (after canceletaions)
d = 900m/2x5 = 90 m