A busy chipmunk runs back and forth along a straight line of acorns that has been set out between its burrow and a nearby tree. At some instant, it moves with a velocity of −1.29 m/s−1.29 m/s . Then, 2.91 s2.91 s later, it moves with a velocity of 1.77 m/s1.77 m/s . What is the chipmunk's average acceleration during the 2.91 s2.91 s time interval?