The function $g$ is a twice differentiable function. Find $f''$ in terms of $g$, $g'$, and $g''$ if $f(x)=xg(x^2)$.
Below, use "g" for $g(x^2)$, "h" for $g'(x^2)$, and "k" for $g''(x^2)$.

$f''(x) =$