$$\frac{2}{x+y} - \frac{a}{(x+y)^2} = \frac{2(x+y)}{(x+y)^2} - \frac{a}{(x+y)^2} = \frac{2x+2y-a}{(x+y)^2}$$
Ответ: $$\frac{2x+2y-a}{(x+y)^2}$$