`x^2 + 4x - 12 = 0`
`⇔x^2 + 6x - 2x - 12=0`
`⇔(x^2 + 6x) - (2x + 12)=0`
`⇔x(x + 6) - 2(x + 6)=0`
`⇔(x + 6)(x - 2)=0`
`⇔`\(\left[ \begin{array}{l}x+6=0\\x-2=0\end{array} \right.\)
`⇔`\(\left[ \begin{array}{l}x=-6\\x=2\end{array} \right.\)
Vậy `x = {-6 ;2}`