A tennis ball with a speed of 8.0 m/s is thrown perpendicularly at a wall. After striking the wall, the ball rebounds in the opposite direction with a speed of 6.0 m/s. If the ball is in contact with the wall for 0.018 s, what is the average acceleration of the ball while it is in contact with the wall? Take the initial direction of motion to be the positive direction. (Indicate the direction of the acceleration by the sign of your answer.)