A bicycle rider sees a car up ahead, and slows down to avoid a collision. He starts off traveling at 15 m s and slows down to 5 m s in 7.5 s. Find the acceleration of the bike rider.

1.333333333 m/s ^2 is the acceleration

Your given date is time=7.5s, Final Velocity=5m/s and Initial velocity=15m/s.

Use the formula a=vf-vi/t. Final velocity minus initial velocity divided by time. In this case you will get a negative answer which means acceleration is in opposite direction which is called deceleration.