a car travels at 25m/s on a slick road takes 4 second to come to a complete stop. what was the car's average acceleration over 4 second?

think about it. A deceleration of 2 m/s^2 means that the velocity decreased by 2 m/s each second.

So, here, the velocity changed by -25 m/s in 4 seconds.

Look at the units. How do you get from m/s to m/s^2?