3x^2y-[2xy^2-4(1/2xy-3/4x^2y)+xy]+3xy^2
=3x^2y-2xy^2+2xy-3x^2y-xy+3xy^2
=xy^2+xy
由x=3,y=-1/3得：
原式=3*1/9-3*1/3=-2/3