m^2+2mn=9,
n^2—mn=4,
1式+2式*2得
m^2+2n^2=9+4*2
m^2+2n^2=17
1式*2-2式*3得
2(m^2+2mn)-3(n^2—mn)=9*2-3*4
2m^2+4mn-3n^2+3mn=18-12
2m^2+7mn-3n^2=6