`a)x(x+2)-3x-6=0`
`<=>x(x+2)-3(x+2)=0`
`<=>(x+2)(x-3)=0`
`=>` \(\left[ \begin{array}{l}x+2=0\\x-3=0\end{array} \right.\)
`=>` \(\left[ \begin{array}{l}x=-2\\x=3\end{array} \right.\)
`b)x^2+x-(x+1)=0`
`<=>x(x+1)-(x+1)=0`
`<=>(x+1)(x-1)=0`
`=>` \(\left[ \begin{array}{l}x+1=0\\x-1=0\end{array} \right.\)
`=>` \(\left[ \begin{array}{l}x=1\\x=-1\end{array} \right.\)