2, Here is the steps

First divide into three group one 1', group two 6, group three 6
If 6 = 6, then the grope one is different than others, find it
If not we know group one is good one use it later
Then take a ball from each side a time, if two side isn't same, then the one is different is still in the 天平，if the both side same then the different one is one of the ball just take out from 天平。
step two use group 1 to compare the two ball s just separate from the good.