$x(x-2) = 4x - 8$
$⇔ x² - 2x - 4x + 8 = 0$
$⇔ x² - 6x + 8 = 0$
$⇔ x² - 2x - 4x + 8 = 0$
$⇔ x( x -2 ) - 4 ( x - 2 ) = 0$
$⇔ ( x - 2) ( x - 4 ) = 0$
⇔ \(\left[ \begin{array}{l}x-2=0\\x-4=0\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=2\\x=4\end{array} \right.\)
$S =$ { $2;4$ }