Đáp án:
$\begin{array}{l}
a)3a + 3b - {a^2} - ab\\
= 3\left( {a + b} \right) - a\left( {a + b} \right)\\
= \left( {a + b} \right)\left( {3 - a} \right)\\
b)\\
{x^2} + x + {y^2} - y - 2xy\\
= \left( {{x^2} - 2xy + {y^2}} \right) + \left( {x - y} \right)\\
= {\left( {x - y} \right)^2} + \left( {x - y} \right)\\
= \left( {x - y} \right)\left( {x - y + 1} \right)\\
c) - {x^2} + 7x - 6\\
= - {x^2} + x + 6x - 6\\
= - x\left( {x - 1} \right) + 6\left( {x - 1} \right)\\
= \left( {x - 1} \right)\left( { - x + 6} \right)
\end{array}$
