subract the sum of m^2n - 2mn^2 and 2m^2n - m - n^3 + 2mn^2 from m^3 - n^3
$$\\(m^3 - n^3)-( m^2n - 2mn^2 )-( 2m^2n - m - n^3 + 2mn^2) \\\\ =m^3 - n^3 -m^2n + 2mn^2 -2m^2n + m + n^3 - 2mn^2 \\\\ =m^3 -m^2n -2m^2n+ 2mn^2 - 2mn^2 + m \\\\ =m^3 -3m^2n + m \\\\$$
