Final answer:
The change in velocity of the baseball is -70 m/s, indicating a direction opposite to the initial direction of travel.
Step-by-step explanation:
The change in velocity of a baseball that is initially traveling at 30 m/s and then hit by a bat to leave at a velocity of -40 m/s, is calculated by taking the final velocity and subtracting the initial velocity. In this case, the change in velocity (final - initial) would be (-40 m/s) - (30 m/s), which equals -70 m/s. This negative sign indicates the direction of the velocity change is opposite to the initial direction of the baseball.