I'm assuming for g(x) you mean g(x) = x^2+3 for [tex]g(x)=x^2+3[/tex].
Going with that...
[tex]\begin{aligned}f(g(x)) &= f\left( x^2+3 \right)~~~~~\text{plug $x^2+3$ in for $x$ in $f(x)$}\\[0.5em]&= 2\cdot ( x^2+3 )~~~~~\text{so $2x$ became $2(x^2+3)$}\\[0.5em]&= 2x^2+6\endaligned}[/tex]
Just treat the enter g(x) function as a single input for f(x).
