`a)(x+1).(y-2) =0`
⇔\(\left[ \begin{array}{l}x+1=0 \\x=-1\end{array} \right.\) ⇔\(\left[ \begin{array}{l}y-2=0\\y=2\end{array} \right.\)
`b)(x-5).(y-7) =1`
⇔\(\left[ \begin{array}{l}x-5=1\\x=6 \end{array} \right.\) ⇔ \(\left[ \begin{array}{l}y-7=1\\x=8\end{array} \right.\)