A rolling ball moves from x1=8.4cm to x2=-4.2cm during the time from t1=3.0s to t2=6.1s. what is its average velocity over this time interval?

The average velocity would simply be the ratio of the change in distance over change in time, that is:

average velocity = (x2 - x1) / (t2 - t1)

average velocity = (-4.2 cm - 8.4 cm) / (6.1 s - 3.0 s)

average velocity = - 4.06 cm/s

Taking the absolute value:

4.06 cm/s