│x-2a│+(y+3)^2=0
必须x-2a=0,y+3=0
即x=2a,y=-3
A=2x^2-3xy+y^2-x+2y
=(2x-y)(x-y)-(x-2y)
B=4x^2-6xy+2y^2-3x-y
=2(2x^2-3xy+y^2)-3x-y
=2(2x-y)(x-y)-3x-y
B-2A=2(2x-y)(x-y)-3x-y-2(2x-y)(x-y)-2(x-2y)
=-3x-y-2(x-2y)
=-3x-y-2x+4y
=3y-x
=-9-2a=a
所以a=-3
x=2a=-6,y=-3
则A=(2x-y)(x-y)-(x-2y)
=(-12+3)(-6+3)-(-6+6)
=9*3
=27