Tìm x , biết: a) x² – 2 ( x + 3 ) = x – 6

       x² – 2 ( x + 3 ) = x – 6

<=> x² – 2x – 6 = x – 6

<=> x² – 2x – x = -6 + 6

<=> x² – 3x = 0

<=> x ( x – 3 ) = 0

<=> \(\left[ \begin{array}{l}x=0\\x-3=0\end{array} \right.\) 

<=> \(\left[ \begin{array}{l}x=0\\x=3\end{array} \right.\) 

Vậy S = {0 ; 3}