Suppose that a car traveling to the west (-x direction) begins to slow down as it approaches a traffic light. Which statement concerning its acceleration?

Its acceleration is positive

Explanation:

As the car is moving in the negative x-direction than after applying brake then there will be a decrease in the acceleration but in the opposite direction.

As decreasing acceleration consider to be negative but the car is moving in negative direction which means increasing acceleration is negative by sign convention but by applying brake acceleration decrease but in opposite direction than it will give positive value of acceleration.