-3x^(3m+1)y^2n*7x^(n-6)y^(-3-m)=-21*x^(3m+n-5)*y^(2n-m-3)与单项式2x^4y是同类项∴x与y的次数相同3m+n-5=4 2n-m-3=1 m=2,乎册裂岁闭姿喊n=3