$$\frac{b}{(x-y)^3} + \frac{10}{3(x-y)^2} = \frac{3b}{3(x-y)^3} + \frac{10(x-y)}{3(x-y)^3} = \frac{3b+10x-10y}{3(x-y)^3}$$
Ответ: $$\frac{10x-10y+3b}{3(x-y)^3}$$