Question

a car traveling at 30 m/s starts to decelerate steadily. it comes to a complete stop in 12 seconds. what is its acceleration?

Answer 1

Just divide -30 m/s (the change in velocity) by 12 seconds (the time required for the car to come to a stop):

-30 m/s
----------- = - 2,5 m / (sec)^2              (answer)
 12 sec