题目

Peter walked up the street. All he wanted to do was play basketball with Sam. But his father told him to return his sister’s books to the library on his way to Sam’s house.In his idea, books and summer vacation can never go together. He thought he had read enough during the school year. So he would never enter a public library. Today he’d just dropped the books off in the outside return box, but it was locked. He had to go inside to return them.Peter walked inside the building a few minutes before closing time. He put the books into the return box. After a quick drink of water and a shor先化简，再求值：2（a+）（a-）-a（a-6）+6，其中a=-1．