`x^2-x-6=0`

`⇔x^2-3x+2x-6=0`

`⇔x(x-3)+2(x-3)=0`

`⇔(x+2)(x-3)=0`

`⇔` \(\left[ \begin{array}{l}x+2=0\\x-3=0\end{array} \right.\) 

`⇔` \(\left[ \begin{array}{l}x=-2\\x=3\end{array} \right.\) 

Vậy `S={-2;3}`

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`x^2-6x+8=0`

`⇔x^2-4x-2x+8=0`

`⇔x(x-4)-2(x-4)=0`

`⇔(x-2)(x-4)=0`

`⇔` \(\left[ \begin{array}{l}x-2=0\\x-4=0\end{array} \right.\) 

`⇔` \(\left[ \begin{array}{l}x=2\\x=4\end{array} \right.\) 

Vậy `S={2;4}`

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`x^2-2x-8=0`

`⇔x^2-4x+2x-8=0`

`⇔x(x-4)+2(x-4)=0`

`⇔(x+2)(x-4)=0`

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

`⇔` \(\left[ \begin{array}{l}x=-2\\x=4\end{array} \right.\) 

Vậy `S={-2;4}`

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`x^2-2x-8=0`

`⇔x^2-4x+2x-8=0`

`⇔x(x-4)+2(x-4)=0`

`⇔(x+2)(x-4)=0`

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

`⇔` \(\left[ \begin{array}{l}x=-2\\x=4\end{array} \right.\) 

Vậy `S={-2;4}`

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`x^2-7x+12=0`

`⇔x^2-4x-3x+12=0`

`⇔x(x-4)-3(x-4)=0`

`⇔(x-3)(x-4)=0`

`⇔` \(\left[ \begin{array}{l}x-3=0\\x-4=0\end{array} \right.\) 

`⇔` \(\left[ \begin{array}{l}x=3\\x=4\end{array} \right.\) 

Vậy `S={3;4}`

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`x^2-x-12=0`

`⇔x^2-4x+3x-12=0`

`⇔x(x-4)+3(x-4)=0`

`⇔(x+3)(x-4)=0`

`⇔` \(\left[ \begin{array}{l}x+3=0\\x-4=0\end{array} \right.\) 

`⇔` \(\left[ \begin{array}{l}x=-3\\x=4\end{array} \right.\) 

Vậy `S={-3;4}`