Assuming the acceleration to be uniform and applying equations of motion
v=u-at where v is the final velocity=0 since the car stops. u=54Km/h=15 m/sec the initial velocity. Here acceleration is negative or it is retardation. Therefore
0=u-at
u=at
t is the time, which is 5sec
15=a x 5
a=15/5=3m/s2