`(x+2)^2=x+2`
`⇔x^2+4x+4=x+2`
`⇔x^2+4x+4-(x+2)=0`
`⇔x^2+4x+4-x-2=0`
`⇔x^2+3x+2=0`
`⇔x^2+x+2x+2=0`
`⇔x(x+1)+2(x+1)=0`
`⇔(x+1)(x+2)=0`
⇔\(\left[ \begin{array}{l}x+1=0\\x+2=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=-1\\x=-2\end{array} \right.\)
Vậy `S={-1;-2}`
`7x(x-y)-21y(y-x)`
`=7x(x-y)+21y(x-y)`
`=(x-y)(7x+21y)`
`=7(x-y)(x+3y)`