26. $$4m(n-m)^2 - 16m^2(m-n) = 4m(n-m)^2 + 16m^2(n-m) = 4m(n-m)((n-m) + 4m) = 4m(n-m)(n+3m)$$
Ответ: $$4m(n-m)(n+3m)$$