题目

Growing up in Philadelphia, Lieberman started cooking with his stay-at-home dad when he was seven. His food-loving family had two kitchens, and he quickly learned what was the best way to bake his cakes. Lieberman improved his kitchen skills greatly during a year abroad before college, learning from a cook in Italy and studying local specialties（地方特色菜）in Germany, Spain and France. At Yale, he was known for throwing dinner parties, single-handedly frying and baking while mixing drinks for dozens of friends. Just for fun, he and some friends decided to tape a show named Cam若函数f（x）=ax（a＞0，且a≠1）的反函数的图象过点（2，-1），则a=    ．