Here it is given that f(x)=3x and g(x)=1/x
We have to find the domain of (g o f)(x)
Now it is given that f(x) = 3x
and it is also given that g(x) = 1/x
so (g o f)(x) = g( f(x) ) = g( 3x )
which comes out to be 1 / 3x
The domain of the expression is all the real numbers except where the expression is undefined so the domain of the given expression is all real numbers except 0.