x3 - x2 - 8x + 12 = 0
=>x3+3x2-4x2-12x+4x+12=0
=> (x3+3x2)-(4x2+12x)+(4x+12)=0
=> x2(x+3)-4x(x+3)+4(x+3)=0
=> (x+3)(x2-4x+4)=0
=> (x+3)(x-2)2=0
=>\(\left[{}\begin{matrix}x+3=0\\x-2=0\end{matrix}\right.\) => \(\left[{}\begin{matrix}x=-3\\x=2\end{matrix}\right.\)
vậy x\(\in\left\{-3;2\right\}\)