Abusy 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?