题目

Young people can have problems with their minds. Some student becomes worried because they have to study very hard. Others have trouble getting on well with people like their parents and classmates.Liu Wei, a junior 2 student from Hefei, could not understand his teacher and was doing badly in his lessons. He became so worried about it that he started to cut his finger with a knife. Another student, 14-year-old Yang Fang from Guangzhou, was afraid of exams. She got very worried in one, and when she looked at the exam paper, she couldn't think of anything to write.A recent report from 已知， ，且，则代数式的值是_____________．