xy(y^2+y)-y^2(xy+2x)-3xy
=xy^3+xy^2-xy^3-2xy^2-3xy
= -xy^2-3xy
= -xy(y+3)
解方程组：x+y=4,x-y=6
得：x=5,y= - 1
代入-xy(y+3)中得:
-5*(-1)*[(-1)+3]
=5*2
=10