Question 380273
That means that wherever you have an {{{x}}} in the function {{{f(x)}}} substitute {{{g}}}.
{{{f(g)=1/(x+8)}}}
where 
{{{g=x+8}}}
Substituting,
{{{f(g)=1/g}}}
So if you work backwards and substitute {{{x}}} for every occurence of {{{g}}} then,
{{{highlight(f(x)=1/x)}}}