Dini is riding her bike eastward at a constant 9.5 m/s when a car pulls out of a side street a short distance in front of her. Dini applies her brakes and manages to stop in 2.5 s, just in time to avoid running into the car. Calculate Dini's average acceleration.

Question

The answer is -3.8 m/s^2 [E], but my teacher said that the final answer is 3.8 m/s^2 [W]. Why is it west when she's still going in the same direction, but just slowing down?