The change in velocity of the tennis ball thrown towards the wall and returning is -5 m/s, where the negative sign denotes a reversal in the ball's direction.
The subject of this question is Physics, and it appears to fit into the High School grade level. It involves concepts of velocity and momentum, which are typically covered in high school physics curricula. To determine the change in velocity of a tennis ball thrown at an initial velocity of 3 m/s and returning at 2 m/s, one must perform vector subtraction because velocity is a vector quantity.
The change in velocity (Δv) is calculated as the final velocity (vf) minus the initial velocity (vi). Assuming a one-dimensional motion and taking the direction towards the wall as positive, the change in velocity would be -2 m/s - 3 m/s, yielding a Δv of -5 m/s. The negative sign indicates that the direction of the velocity change is opposite to the direction towards the wall.