{（m＋n）²—4（m—n）²}÷（3n—m）
=（m^2+2mn+n^2-4m^2+8mn-4n^2)/(3n-m)
=-(3m^2-10mn+3n^2)/(3n-m)
=-(3m-n)(m-3n)/(3n-m)
=3m-n