\[\frac{a + 3}{1 - a} \cdot \left( \frac{a}{a - 3} + \frac{3 - a}{a + 3} \right)^{- 3} =\]
\[= \frac{9a - 9}{(1 - a)(a - 3)} =\]
\[= \frac{9 \cdot (a - 1)}{- (a - 1)(a - 3)} = \frac{- 9}{a - 3}\]