Answer
The domain is $(0,18)$.
Work Step by Step
From the volume equation:
$$V=x(36-2x)(36-2x)$$ We must have: $$\begin{aligned}
\begin{cases}
x&>0\\
36-2x&>0
\end{cases}
\end{aligned}$$Therefore $$\begin{aligned}
\begin{cases}
x&>0\\
x&<18
\end{cases}
\end{aligned}$$
Thus, the domain is $(0,18)$.
