15) $$\frac{14m-n}{xy} - \frac{2n-3m}{xy} = \frac{14m-n - (2n-3m)}{xy} = \frac{14m-n - 2n + 3m}{xy} = \frac{17m - 3n}{xy}$$
Ответ: $$\frac{17m-3n}{xy}$$