题目

On the evening of June 21, 1992, a tall man with brown hair and blue eyes entered the beautiful hall of the Bell Tower Hotel in Xi’an with his bicycle. The hotel workers received him and telephoned the manager, for they had never seen a bicycle in the hotel hall before though they lived in “the kingdom of bicycles.”Robert Friedlander, an American, arrived in Xi’an on his bicycle trip across Asia which started last December in New Delhi, India.When he was 11, he read the book Marco Polo and made up his mind to visit the Silk Road. Now, after 44 years , he was on the Silk Road in Xi’an一个实数的两个平方根分别是a+3和2a-5，则这个实数是________．