## Algebra 2 (1st Edition)

$f(f(x))=f(3x^{-1})=f(\frac{3}{x})=3(\frac{3}{x})^{-1}=3\frac{x}{3}=x$ The denominator of this function's first term cannot be $0$, thus $x\ne0$, and the domain is all real numbers apart from $x=0$.