Đáp án:
Câu a xem lại đề bài
$\begin{array}{l}
b)9x\left( {x + y} \right) + 3{\left( {y + x} \right)^2}\\
= 3.\left( {x + y} \right).\left( {3x + x + y} \right)\\
= 3.\left( {x + y} \right).\left( {4x + y} \right)\\
c){\left( {3x - 1} \right)^2} - \left( {3x - 1} \right).5\\
= \left( {3x - 1} \right).\left( {3x - 1 - 5} \right)\\
= \left( {3x - 1} \right).\left( {3x - 6} \right)\\
= 3.\left( {3x - 1} \right).\left( {x - 2} \right)
\end{array}$