PTĐTTNT : a)x^2-xy-6y^2 b)x^3-3x^2-6x+8=0

$a)x^2-xy-6y^2\\ =x^2-3xy+2xy-6y^2 \\ =x(x-3y)+2y(x-3y) \\ =(x+2y)(x-3y) \\ b)x^3-3x^2-6x+8=0\\ \to x^3-x^2-2x^2+2x-8x+8=0\\ \to x^2(x-1)-2x(x-1)-8(x-1) =0 \\\to (x-1)(x^2-2x-8)=0\\\to(x-1)(x^2+2x-4x-8)=0\\\to(x-1)[x(x+2)-4(x+2)]=0\\\to(x-1)(x+2)(x-4)=0$\(\to\left[ \begin{array}{l}x-1=0\\x+2=0\\x-4=0\end{array} \right.\\\) \(\to\left[ \begin{array}{l}x=1\\x=-2\\x=4\end{array} \right.\)