Acceleration is defined as the rate of change of velocity. As velocity is a vector quantity, so acceleration is also a vector quantity.
If a car starts from rest and attains some velocity after some time, then car is accelerating and the firection of acceleration is same as the direction of velocity.
If a car is moving and after applying the brakes it come to rest it means the motion of car has negative acceleration which means the direction of velocity and the direction of acceleration is opposite to each other.
Acceleration is a vector because it has magnitude and direction
Example : a Car stopping at the rate of 5 m/s. In this example, the direction of acceleration is the opposite of the direction of velocity, and the magnitude is 5 m/s
