б) $$\frac{c}{\sqrt{c} + 9} - \frac{81}{\sqrt{c} + 9} = \frac{c - 81}{\sqrt{c} + 9} = \frac{(\sqrt{c} - 9)(\sqrt{c} + 9)}{\sqrt{c} + 9} = \sqrt{c} - 9$$.
Ответ: $$\sqrt{c} - 9$$