题目

Studying too much? Most people work or study from Monday to Friday. The weekend is usually a time of rest. But today’s Chinese teenagers can hardly rest during the weekends. According to a survey, 24% of the Grade 8 students in Beijing have classes at the weekend．Over 40% of the middle school students have less than eight hours’ sleep each night because of study. And this doesn’t happen only in Beijing. Ji Chunying is a Grade 9 student in Guangzhou. The 14-year-old girl has to get up at 6:45am on Saturday. Then she has classes all the day. On Sunday, she goes to extra（额外）classe设a=，则二项式(a-)6展开式中含x2项的系数是  _