(x-y)(-y-x)-(3x三次方y-6x²y²-3xy三次方)/(-3xy)怎样化简?

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(x-y)(-y-x)-(3x三次方y-6x²y²-3xy三次方)/(-3xy)怎样化简?

(x-y)(-y-x)-(3x³y-6x²y²-3xy³)/(-3xy)
=-(x-y)(x+y)-3xy(x²-2xy+y²)/(-3xy)
=-(x²-y²)+(x²-2xy+y²)
=-x²+y²+x²-2xy+y²
=2y²-2xy