m^2+2mn+2n^2-6n+9=0
m^2+2mn+n^2+n^2-6n+9=0
(m^2+2mn+n^2)+(n^2-6n+9)=0
(m+n)^2+(n-3)^2=0
m+n=0,n-3=0
m=-n,n=3
所以
m=-3,n=3