3A+6B
=3(2x^2+4xy-2x-3)+6(-x^2+xy+2)
=6x^2+12xy-6x-9-6x^2+6xy+12
=18xy-6x+3
=x(18y-6)+3
多项式的值与x的取值无关,则x项系数=0
18y-6=0
y=1/3