1. Convert km/h to m/s:

* 500 km/h * (1000 m/km) * (1 h/3600 s) = 138.89 m/s

2. Determine the change in velocity:

* Initial velocity (vi) = 138.89 m/s

* Final velocity (vf) = 0 m/s

* Change in velocity (Δv) = vf - vi = 0 - 138.89 = -138.89 m/s

3. Calculate the acceleration:

* Acceleration (a) = Δv / Δt

* a = -138.89 m/s / 20 s = -6.94 m/s²

Answer: The acceleration of the vehicle is -6.94 m/s². The negative sign indicates that the vehicle is decelerating (slowing down).