(ax^2-2xy+6y^2)-(-ax^2+bxy+5y^2)=6x^2-9xy+cy^2
2ax^2-(2+b)xy+y^2=6x^2-9xy+cy^2
2a=6 a=3
-2-b=-9 b=7
c=1
选 A.3,7,1