（ax^2-2xy+y^2)-(-ax^2+bxy+2y^2)
=ax^2-2xy+y^2+ax^2-bxy-2y^2
=2ax^2-(b+2)xy-y^2
=6x^2-9xy+cy^2
所以2a=6
b+2=9
-1=c
所以
a=3
b=7
c=-1