$$\frac{3}{2(x+y)} + \frac{b}{(x+y)^2} = \frac{3(x+y)}{2(x+y)^2} + \frac{2b}{2(x+y)^2} = \frac{3x+3y+2b}{2(x+y)^2}$$
Ответ: $$\frac{3x+3y+2b}{2(x+y)^2}$$