题目

You have a first name to tell who you are. You have a last name to show which family you are from. But long time ago, people didn’t have last names. Because there was no need. Later they had to （不得不）have last names because there were so many Jacks, too many Bills, and too many Toms. People started（开始）to ask, which Jack? Which Bill? Which Mary? So if a Jack was a doctor, he became Jack Doctor. If a Bill was a son of Peter, he became Bill Peterson. If a Mary lived near the bush（灌木丛）, she became Mary Bush. This is how some last names started long long ago.【1】One15．某同学误将“A-B”看成求“A+B”，结果求出的答案是3x2-2x+5，已知A=4x2-3x-6，请正确求出A-B．