In this case, the function is defined for all $x$ such that $g(x) \geq 0$. So, we solve the inequality $540 - 0.2x^2 \geq 0$ for $x$:
\begin{align*}
540 - 0.2x^2 &\geq 0 \\
-0.2x^2 &\geq -540 \\
x^2 &\leq 2700 \\
x &\leq \sqrt{2700} = 30\sqrt{3}
\end{align*}
So,
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