(2X^2+AX-Y+6)-(2BX^2-3X+5Y+1)
=2X^2+AX-Y+6-2BX^2+3X-5Y-1
=(2-2B)X^2+(A+3)X-6Y+5
值与字母X的取值无关则由X的项系数为0
2-2B=0,A-3=0
A=-3,B=1
1/3A^3-2B^2-1/4A^3+2B^2
=1/12A^3
=-27/12
=-9/4