A car slows down from 23 m/s to rest in a distance of 85m. what was its acceleration, assumed constant?

In this case, the car slow down from 23m/s into rest which means 0m/s. That mean the average speed of the car would be: (23m/s + 0 ms) / 2 = 11.5m/s.

If the car moved for 85m with that average speed, then the time car moves would be:

85m / (11.5m/s) = 7.39s

The car moves for 7.39 second, losing velocity from 23m/s into 0m/s. Then the acceleration would be:

(23m/s - 0m/s) / 7.39s = 3.11 second