Given that $f(3)=5$ and $f(3x)=f(x)+2$ for all $x > 0$, find $f^{-1}(11)$.
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Given that $f(3)=5$ and $f(3x)=f(x)+2$ for all $x > 0$, find $f^{-1}(11)$.
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
81
Given that $f(3)=5$ and $f(3x)=f(x)+2$ for all $x > 0$, find $f^{-1}(11)$.
train