[填空题]
A rolling ball moves frf-e ju ft/bav 34)ry:pkl3e7towq+lj m+9tt kt ;mpq3*m $x_1=8.8 \mathrm{~cm}$ 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)
