Answer
See below.
Work Step by Step
$g(f(x))=g(3x^{-1})=g(\frac{3}{x})=2\frac{3}{x}-7=\frac{6}{x}-7$
The denominator of this function's first term cannot be $0$, thus $x\ne0$. Therefore, the domain is all real numbers apart from $x=0$.
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