posted by Tig .
A dog running in the +y-direction at 6.6m/s hears its owner call from behind, and 3.0secs later it's running at the same speed in the opposite direction.
Compute the dog's change in velocity.
Compute its average acceleration.
Change in velocity = -3 - 6.6 = -9.3 m/s
Acceleration = (-9.3 m/s)/3.0s = -3.1 m/s^2
(minus means backwards compared to the +y direction)