\[\boxed{\text{751\ (751).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[1)\ \frac{a - 3}{a^{2} - 3a + 9} - \frac{6a - 18}{a^{3} + 27} =\]
\[= \frac{(a - 3)(a + 3) - 6 \cdot (a - 3)}{a^{3} + 27} =\]
\[= \frac{(a - 3)(a + 3 - 6)}{a^{3} + 27} =\]
\[= \frac{(a - 3)(a - 3)}{a^{3} + 27}\]
\[2)\ \frac{(a - 3)(a - 3)}{a^{3} + 27} \cdot \frac{4 \cdot \left( a^{3} + 27 \right)}{5 \cdot (a - 3)} =\]
\[= \frac{4}{5} \cdot (a - 3).\]
\[\textbf{б)}\ \frac{ab^{2} - a^{2}b}{a + b} \cdot \frac{a + \frac{\text{ab}}{a - b}}{a - \frac{\text{ab}}{a + b}} =\]
\[= \frac{\text{ab}(b - a)}{a + b} \cdot \frac{1 + \frac{b}{a + b}}{1 - \frac{b}{a + b}} =\]
\[= \frac{\text{ab}(b - a)}{a + b} \cdot \frac{\frac{a - b + b}{a - b}}{\frac{a + b - b}{a + b}} =\]
\[= \frac{\text{ab}(b - a)}{a + b} \cdot \frac{a}{a - b} \cdot \frac{a + b}{a} =\]
\[= - ab.\]