所有含x项系数为0
(2x2+ax-y+6)-2bx2-3x-5y-1
=(2-2b)x^2+(a-3)x-6y+5
2-2b=0,b=1
a-3=0,a=3
1/3a3-2b2-(1/4a3-3b2)
=3^3/3-2-3^3/4+3
=13/4