A car traveling at 24 m/s starts to decelerate steadily. It comes to a complete stop in 15 seconds.

A = delta v over delta t delta v is calculated with final velocity less initial velocity so delta v is 0 - 24 that is - 24m/s and delta t is calculated with final time less initial time as initial always is 0 then is 15 - 0 that is 15 then a = - 24m/s over 15 that is - 1,6 then the "acceleration" (on real is a desacceleration) is - 1,6m/s^2 (remember that is squared.)