Functions

If \(f: R \rightarrow R\) and \(g: R \rightarrow R\) are two functions defined by \(f(x)=a x+b(a \neq 0), \forall x \in R\) and \(g(x)=c x^3+d(c \neq 0), \forall x \in R\), then \((f \circ g)^{-1}(x)\) is equal to