15.55
a) $$\frac{a}{\sqrt{a} - 3} - \frac{9}{\sqrt{a} - 3} = \frac{a - 9}{\sqrt{a} - 3} = \frac{(\sqrt{a} - 3)(\sqrt{a} + 3)}{\sqrt{a} - 3} = \sqrt{a} + 3$$.
Ответ: $$\sqrt{a} + 3$$