Ta có: \(\sqrt{x^6}=3\)
\(\Leftrightarrow\sqrt{\left(x^3\right)^2}=3\)
\(\Leftrightarrow\left|x^3\right|=3\)
\(\Leftrightarrow\left[{}\begin{matrix}x^3=3\\x^3=-3\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=\sqrt[3]{3}\\x=\sqrt[3]{-3}\end{matrix}\right.\)
Vậy: \(S=\left\{\sqrt[3]{3};\sqrt[3]{-3}\right\}\)