Data:
Time (t) = 5 seconds
Final Velocity (v) = 0 [as the car comes to rest]
Initial velocity (u) = 20 m/s
Acceleration = ? (Negative because it is braking)
\(Acceleration = \frac{v-u}{t}\)
\(Acceleration = \frac{0-20}{5}\)
\(\boxed{Acceleration = - 4m/s^2}\)