若有理数m,n满足m-2n=4,2m-n=3,则m+n等于

若有理数m,n满足m-2n=4,2m-n=3,则m+n等于

m-2n=4
2m-n=3
2m-4n=8
2m-n=3
-3n=5
n=-5/3
m=4-2*5/3=2/3
m+n=-5/3+2/3=-1

解由
m-2n=4,...............①
2m-n=3.................②
由②-①
得m+n=-1若有理数m,n满足m-2n=4,2m-n=3,则m+n等于
怎么算的？晕死。