[填空题]
A rolling ball moves fd wbuabwg p4i()z*0q. r6 m1 q ys au7)j6vvn5awsuwx5/om $x_1=8.8 \mathrm{~cm}$ to $x_2=-2.5 \mathrm{~cm}$ during the time from $t_1=4.0 \mathrm{~s}$ to $t_2=12.0 \mathrm{~s}$. What is its average velocity? $cm/s$ (only fill in numerical value please)
