To test the quality of a tennis ball, you drop it onto the floor from a height of $4.00 \mathrm{~m}$. It rebounds to a height of $2.00 \mathrm{~m}$. If the ball is in contact with the floor for $12.0 \mathrm{~ms}$, (a) what is the magnitude of its average acccleration during that contact and (b) is the average acceleration up or down?