Question

How long will it take for a car to accelerate to come to a stop if it is traveling at 20m/s and the car has a maximum deceleration of 6m/s

Answer 1

When the car comes to a stop, the final velocity must be 0 m/s.
Since the car js decelerating in a forward direction, acceleration must be negative.

final v = initial v + a•t
0 = 20 + (-6)t
t = 3.33s