(x-4)(x+3x+1)=0
(x-4)(4x+1)=0
x-4=0 hoặc 4x+1=0
x=4 hoặc x=\(\frac{-1}{4}\)
x(x-4) +3(x-4)(3x+1)=0
\(\Leftrightarrow\left(x-4\right)\left(4x+1\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x-4=0\\4x+1=0\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=4\\x=-\frac{1}{4}\end{matrix}\right.\)
vậy: S=\(\left\{-\frac{1}{4};4\right\}\)