（x^m÷x^2n）÷x^2m-n
=x^(m-2n-2m+n)
=x^(-m-n)
与2x^3是同类项
-m-n=3
m+n=-3 (1)
m+5n=2 (2)
(2)-(1)
4n=5,n=5/4
m=-3-n=-17/4
m^2-25n=-211/16