Taking the downwards vertical direction (towards the ground) as negative, a tennis ball is made to bounce on the ground. Just before hitting the ground, the magnitude of the tennis ball's velocity is 4.2 m/s. After the bounce, it rebounds upwards with a velocity of magnitude 3.9 m/s. The bounce takes place during a time interval of 12.9 ms (milliseconds). Determine the acceleration of the tennis ball during the bounce. be mindful of the sign conventions of the vectors used. Give your answer to 1 decimal place.