$7m(m{n}^3+3n^4-2m^2{n}^2)$

$7n^2(m^{2}n+3m{n}^2-2m^3)$

$m{n}^2(7mn+21n^2-14m^2)$

$7m{n}^2(mn+3n^2-2m^2)$