f(x)=a^x-10a/3
反函数过(-1,2)
f(x)过(2,-1)
f(2)=a^2-10a/3=-1
3a^2-10a+3=0
(3a-1)(a-3)=0
a=1/3,或a=3
f(x)=(1/3)^x-10/9
f(x)=3^x-10