已知 $\mathit{2}\mathit{m}\mathit{-}\mathit{3}\mathit{n}\mathit{=}\mathit{-}\mathit{4}$，则代数式 $\mathit{m}\mathit{(}\mathit{n}\mathit{-}\mathit{4}\mathit{)}\mathit{-}\mathit{n}\mathit{(}\mathit{m}\mathit{-}\mathit{6}\mathit{)}$的值为　   　．