The acceleration is 
Explanation:
The acceleration of an object is the rate of change in velocity of the object.
Mathematically, it is given by

where
u is the initial velocity
v is the final velocity
t is the time taken for the velocity to change from u to v
For the car in this problem, we have:
u = +20 m/s
v = 0 (because the car comes to a stop)
t = 6.5 s (time taken)
Therefore, the acceleration is

Where the negative sign means that the direction of the acceleration is opposite to the direction of the initial velocity.
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