For rectilinear motion having a constant acceleration, the average velocity is meant literally as an average of the initial and final velocity. Generally, the average velocity is equal to the total distance to the total time. However, for constant acceleration, it is also equivalent to the mean of the two velocities. Hence,
average velocity = (0 + 12 m/s)/2 = 6 m/s
Since the motion is heading west, according to conventionality, the velocity should have a negative sign. So, the average velocity is -6 m/s.
