The deceleration of the car is 3 m/s².
Step-by-step explanation:
So the initial velocity of the car is given as 12 m/s. As the car is said to slow down and then it stops after 4 seconds, so the final velocity is zero.
We know that the acceleration acting on any object to stop the motion of that object is termed as deceleration or negative acceleration.
This is because, acceleration is the ratio of change in velocity to time taken for that change in velocity.
So.,


So the deceleration of the car is 3 m/s².