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 \(f(g(x))\)?