Упростим выражение:
$$\frac{x^3y+xy^3}{2(y-x)} \cdot \frac{5(x-y)}{x^2+y^2} = \frac{xy(x^2+y^2)}{2(y-x)} \cdot \frac{5(x-y)}{x^2+y^2} = \frac{5xy(x-y)}{2(y-x)} = \frac{5xy(x-y)}{-2(x-y)} = -\frac{5xy}{2}$$
Подставим значения x = -3 и y = 1/3:
$$-\frac{5(-3)(1/3)}{2} = -\frac{-5}{2} = \frac{5}{2}$$