15) b+2 b = b2-9 9-b2

15) $$ \frac{b+2}{b^2-9} - \frac{b}{9-b^2} = \frac{b+2}{b^2-9} - \frac{b}{-(b^2-9)} = \frac{b+2}{b^2-9} + \frac{b}{b^2-9} = \frac{b+2+b}{b^2-9} = \frac{2b+2}{b^2-9} = \frac{2(b+1)}{(b-3)(b+3)} $$

Ответ: $$\frac{2(b+1)}{(b-3)(b+3)}$$