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