д) \frac{2mn-6m^{2}}{12m^{2}n-4mn^{2}} = \frac{2m(n-3m)}{4mn(3m-n)} = - \frac{2m(3m-n)}{4mn(3m-n)} = - \frac{1}{2n}
Ответ: - \frac{1}{2n}