\[6x + 7y + 42 + xy =\]
\[= xy + 6x + 7y + 42 =\]
\[= x(y + 6) + 7 \cdot (y + 6) =\]
\[= (y + 6)(x + 7).\]
\[Проверка:\]
\[(y + 6)(x + 7) =\]
\[= xy + 7y + 6x + 42 =\]
\[= 6x + 7y + 42 + xy.\ \]