1.  First of all, it means that your original formulation in the post was wrong.

    The correct writing is  fog(x)=2x+3.

    The composition is always denoted as fog.


2.  Then you can write fog(x) = f(g(x)) =  = 2x + 3.

    Replace here  by other variable, for example, t.

    You will get f(t) = 2x + 3.

    But since t = , you have x = , therefore

    f(t) =  = .


    Now you can replace t in both sides by x (it is simply another symbol for the variable), and you will get f(x) = .