Answer
The composite function $f\circ g$ is defined such that
$$f\circ g(x)=f(g(x))$$
and its domain is
$$\mathcal{D}=\{x\in B|g(x)\in A\},$$
where $A$ is the domain of $f$ and $B$ is the domain of $g$.
Work Step by Step
The composite function $f\circ g$ is defined such that
$$f\circ g(x)=f(g(x))$$
i.e. we first evaluate $g$ at $x$ and then we evaluate $f$ at $g(x)$. This means that $x$ has to be in the domain of $g$ and that $g(x)$ has to be in the domain of $f$. We can denote this set as
$$\mathcal{D}=\{x\in B|g(x)\in A\}$$
Where $A$ is the domain of $f$ and $B$ is the domain of $g$.
