A shopper is pushing a cart down a grocery store aisle. Starting from rest, the shopper applies a constant force to the cart for 4.0 s. From t = 4s until t = 4 s until t = 8 s, the shopper applies just enough force to balance the friction on the cart. Finally, the shopper applies a constant force to slow the cart until it comes to rest att = 12 s. The resulting position-time graph is shown below. What is the average acceleration of the shopping cart during its 12s motion?