\[6m^{2}n - 3m^{2} + 2mn^{2} - mn =\]
\[= 3m^{2}(2n - 1) + mn(2n - 1) =\]
\[= (2n - 1)\left( 3m^{2} + mn \right) =\]
\[= m(2n - 1)(3m + n)\]