Đáp án:
$\begin{array}{l}
a)\frac{2}{3}{x^2}y.\left( {3xy - {x^2} + y} \right)\\
= {x^3}{y^2} - \frac{2}{3}{x^4}y + \frac{2}{3}{x^2}{y^2}\\
b)\frac{2}{{x + 3}} - \frac{3}{{x - 3}} + \frac{{3x + 21}}{{{x^2} - 9}}\\
= \frac{{2\left( {x - 3} \right) - 3\left( {x + 3} \right) + 3x + 21}}{{\left( {x - 3} \right)\left( {x + 3} \right)}}\\
= \frac{{2x - 6 - 3x - 9 + 3x + 21}}{{\left( {x - 3} \right)\left( {x + 3} \right)}}\\
= \frac{{2x - 6}}{{\left( {x - 3} \right)\left( {x + 3} \right)}}\\
= \frac{{2\left( {x - 3} \right)}}{{\left( {x - 3} \right)\left( {x + 3} \right)}}\\
= \frac{2}{{x + 3}}
\end{array}$
