\(4\left(x-3\right)=x^2-3x\)
\(\Leftrightarrow4\left(x-3\right)=x\left(x-3\right)\)
\(\Leftrightarrow4\left(x-3\right)-x\left(x-3\right)=0\)
\(\Leftrightarrow\left(x-3\right)\left(4-x\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x-3=0\\4-x=0\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=3\\x=4\end{matrix}\right.\)
Vậy: \(S=\left\{3;4\right\}\)