г) $$\frac{3x + y}{x^2 + xy} - \frac{x + 3y}{y^2 + xy}$$.

г) $$\frac{3x + y}{x^2 + xy} - \frac{x + 3y}{y^2 + xy} = \frac{3x + y}{x(x + y)} - \frac{x + 3y}{y(y + x)} = \frac{y(3x + y) - x(x + 3y)}{xy(x + y)} = \frac{3xy + y^2 - x^2 - 3xy}{xy(x + y)} = \frac{y^2 - x^2}{xy(x + y)} = \frac{-(x^2 - y^2)}{xy(x + y)} = \frac{-(x - y)(x + y)}{xy(x + y)} = \frac{-(x - y)}{xy} = \frac{y - x}{xy}$$.

Ответ: $$\frac{y - x}{xy}$$