Acceleration is defined as the rate of change of velocity. That is,
acceleration, a = dv/dt
Here, the velocity of the car increases from 0 km/h to 100 km/h in 5 seconds. Let us first convert the velocity to m/s units, using the knowledge that 1 km is equal to 1000 m and 1 hour has 3600 seconds.
Thus, 0 km/h = 0 m/s
and , 100 km/h = (100 x 1000) m/(1 x 3600) s = 27.8 m/s
Thus, the acceleration of the car is:
a = (27.8 - 0)(m/s) / 5 s = 5.6 m/s^2
Thus the car has an acceleration of 5.6 m/s^2.
If the car can decelerate at 5 m/s^2, then it would need
t = (v - u)/a = (0 - 27.8) / 5 = 5.56 sec to stop completely.
Hope this helps.