题目

Growing up in the USA, I was ashamed of my mother’s Chinese English. Because of her English, she was often treated unfairly. People in department stores, at banks, and at restaurants did not take her seriously, did not give her good service, pretended not to understand her, or even acted as if they did not hear her.My mother has realized the limitations of her English as well. When I was fifteen, she used to have me call people on phone to pretend I was her. I was forced to ask for information or even to yell at people who had been rude to her. One time I had to call her stockbroker (股票若圆x2+y2=4与圆x2+y2+2ay-6=0（a＞0）的公共弦的长为，则a=    ．