subract the sum of m^2n - 2mn^2 and 2m^2n - m - n^3 + 2mn^2 from m^3 - n^3
(m3−n3)−(m2n−2mn2)−(2m2n−m−n3+2mn2)=m3−n3−m2n+2mn2−2m2n+m+n3−2mn2=m3−m2n−2m2n+2mn2−2mn2+m=m3−3m2n+m