A simple substitution for $x$ . Isn't it?

Geometry Level 3

$\large \begin{cases} f(x) = \ln \left( \dfrac{1+x}{1-x} \right), & \quad f : (-1,1) \to \mathbb{R} \\ g(x) = \dfrac{3x+x^3}{1+3x^2}, & \quad g : \mathbb{R} \to (-1,1) \end{cases}$

For $f(x)$ and $g(x)$ as defined above, what is $f \circ g (x)$ or $f(g(x))$ ?

A simple substitution for xxx. Isn't it?

A simple substitution for $x$ . Isn't it?