首先2m^2+3n^2-mn-3m^2-4n^2+mn+6=（2m^2-3m^3）+(3n^2-4n^2)+(mn-mn)+6=-m^2-n^2+6
m^2+n^2=5,所以-m^2-n^2=-5
所以,-m^2-n^2+6=-5+6=1
所以2m^2+3n^2-mn-3m^2-4n^2+mn+6=1