While traveling along a highway a driver slows from 24 m/s to 15 m/s in 12 seconds. what is the automobile’s acceleration?

Average Acceleration = (change in speed) / (time for the change)

Change in speed = (ending speed) - (starting speed)

                             =   15 m/s - 24 m/s  =  -9 m/s

Acceleration =  (-9 m/s) / (12 sec) =  - 0.75 m/s² .