[填空题]
A rolling ball moves from n g5htqeht t5n/v 7t+o97f /mc(xf3rqs; 3pu id;+cfv$x_1=8.8 \mathrm{~cm}$ 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)
