5) a²-4a/a²-9 - 2a-9/a²-9 =

$$\frac{a^2-4a}{a^2-9} - \frac{2a-9}{a^2-9} = \frac{a^2-4a - (2a-9)}{a^2-9} = \frac{a^2-4a - 2a + 9}{a^2-9} = \frac{a^2-6a + 9}{a^2-9} = \frac{(a-3)^2}{(a-3)(a+3)} = \frac{a-3}{a+3}$$

Ответ: (a-3)/(a+3)