(x-2y)(x+3y)-(2x-y)(x-4y)

=x？ +3xy-2xy-6y？ - (2x？ -8xy-yx+4y？ )

=-x？ + 10xy- 10y？

x=- 1，y=2

Original formula = -(- 1)? + 10*(- 1)*2- 10*2? =- 1-20-40=-6 1

Supplement:

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