已知xy²=-2,求-xy(x³y七次方-3x²y五次方-y)

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-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