\[\begin{array}{l}
a)\,\,{x^2} - 2{x^2} + x - x{y^2}\\
= - {x^2} + x - x{y^2}\\
= - x\left( {x + {y^2} - 1} \right).\\
\Rightarrow Xem\,\,lai\,\,\,de\,\,bai.\\
b)\,\,7{x^2} - 12x + 5\\
= 7{x^2} - 7x - 5x + 5\\
= 7x\left( {x - 1} \right) - 5\left( {x - 1} \right)\\
= \left( {x - 1} \right)\left( {7x - 5} \right).
\end{array}\]