2m－{n＋[4n－3(m＋2n)＋6m]－5n}
=2m－{n＋[4n－3m-6n＋6m]－5n}
=2m－{n＋3m-2n－5n}
=2m－{3m-6n}
=2m－3m+6n
=-m+6n
=-2/3+6*4
=23又1/3