m的相反数是2n-8
-m=2n-8
m+2n=8 (1)
n的相反数是3m+1
-n=3m+1
3m+n=-1 (2)
2*(2)-(1)
6m-m=-2-8
m=-2
n=5
m^2+n^2
=(-2)^2+5^2
=4+25
=29