\(\sqrt{2}x^2+6x=0\)
\(\Leftrightarrow x\left(\sqrt{2}x+6\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=0\\\sqrt{2}x+6=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=0\\x=-3\sqrt{2}\end{matrix}\right.\)
Vậy \(S=\left\{0;-3\sqrt{2}\right\}\)