原式=x^2(m+n)*[-x^3(m-n)]-x^(2m-n+3)
=x^(2m+2n)*[-x^(3m-3n)]-x^(2m-n+3)
=-x^(2m+2n+3m-3n)-x^(2m-n+3)
=-x^(5m-n)-x^(2m-n+3)