here is how function notations work. whatever shows up in the brackets, you replace x with it (or whatever variable the function is defined for)

example: if

then, ..........i replaced x with 1

..................i replaced x with 3

...............i replaced x with ....(assume is in the domain of g(x) )

what if we had another function?

then, ...........i replaced the x in g(x) with h(x)

and

............i replaced the x in h(x) with g(x)

now, can you do your problem?