Đáp án:
$\begin{array}{l}
{x^2} + 2xy + {y^2} - x - y - 12\\
= {\left( {x + y} \right)^2} - \left( {x + y} \right) - 12\\
= {\left( {x + y} \right)^2} - 4\left( {x + y} \right) + 3\left( {x + y} \right) - 12\\
= \left( {x + y} \right)\left( {x + y - 4} \right) + 3\left( {x + y - 4} \right)\\
= \left( {x + y - 4} \right)\left( {x + y + 3} \right)
\end{array}$
