题目

阅读理解。One day Mrs Wilson took Trudy and Ben to go shopping. They went to the supermarket in the new shopping mall(购物中心). “Why do you buy things here, Mum?” Trudy asked.  “Because they are cheaper than those at the corner store,” Mrs Wilson said. “Help me check(检查) the prices.” The Wilsons were not wealthy(富有的) and Mrs Wilson was always careful with her money. She looked carefully at the prices of things. She bought some groceries(杂货) in the supermarket. When they got home, the children said, “We don't think you saved（省） money by going to the 经过点（2，-1），且与直线x+y-5=0垂直的直线方程是    ．