Ответ:

[tex]\displaystyle \frac{3}{x+y}[/tex]

Объяснение:

[tex]\displaystyle \frac{x-y}{xy}:\frac{x^2-y^2}{3xy}=\frac{x-y}{xy}:\frac{(x-y)(x+y)}{3xy}=\\\\\\\\\frac{x-y}{xy}\cdot \frac{3xy}{(x-y)(x+y)}=\frac{3}{x+y}[/tex]