-xy(x³y七次方-3x²y五次方-y)=-xy[(x³y^6)y-3(x²y^4)y-y]=-xy[(xy²)³y-3(xy²)²y-y]=-xy[(-2)³y-3(-2)²y-y]=-xy(-8y-12y-y)=21xy²=21×(-2)=-42