Đáp án:
$\begin{array}{l}
a)14{x^2}y - 21x{y^2} + 28{x^2}{y^2}\\
= 7xy\left( {2x - 3y + 4xy} \right)\\
b){x^2} - 6x + 9 - {y^2}\\
= {\left( {x - 3} \right)^2} - {y^2}\\
= \left( {x - 3 - y} \right)\left( {x - 3 + y} \right)\\
c)x\left( {x + y} \right) - 3x - 3y\\
= x\left( {x + y} \right) - 3\left( {x + y} \right)\\
= \left( {x + y} \right)\left( {x - 3} \right)\\
d){x^2} + {\left( {x - 2} \right)^2} - 4\\
= {x^2} - 4 + {\left( {x - 2} \right)^2}\\
= \left( {x - 2} \right)\left( {x + 2} \right) + {\left( {x - 2} \right)^2}\\
= \left( {x - 2} \right)\left( {x + 2 + x - 2} \right)\\
= 2x.\left( {x - 2} \right)\\
e)3\left( {x - y} \right) - 5x\left( {x - y} \right)\\
= \left( {x - y} \right)\left( {3 - 5x} \right)\\
f)x\left( {y - 1} \right) - y\left( {y - 1} \right)\\
= \left( {y - 1} \right)\left( {x - y} \right)\\
g)10x\left( {x - y} \right) - 8y\left( {x - y} \right)\\
= 2\left( {x - y} \right)\left( {5x - 4y} \right)\\
h)2{x^2} + 5xy - 3{x^3}\\
= x\left( {2x + 5y - 3{x^2}} \right)
\end{array}$
