A car sees another parked car in its path and hits the breaks, it leaves 20m of skid marks and its final velocity when it hits the car in 5m/s. You also are given µ=.8 and know force of friction=µ*normal force.

so you know Vf=5 m/s, Xi= 0, Xf=20 and µ=.8. how do you find the cars deceleration????