\(\left(3x-1\right)^2-5\left(2x+1\right)^2+\left(6x-3\right)\left(2x+1\right)=\left(x-1\right)^2\)
\(\Leftrightarrow\left(9x^2-6x+1\right)-5\left(4x^2+4x+1\right)+12x^2+6x-6x-3=x^2-2x+1\)
\(\Leftrightarrow9x^2-6x+1-20x^2-20x-5+12x^2+6x-6x-3=x^2-2x+1\)
\(\Leftrightarrow9x^2-6x-20x^2-20x+12x^2+6x-6x-x^2+2x=1-1+5+3\)
\(\Leftrightarrow-24x=8\Leftrightarrow x=\frac{-1}{3}\)
Vậy S = \(\left\{\frac{-1}{3}\right\}\)