原式=(21x²-14xy-7x-6xy+4y²+2y)-(36x²+24xy+27xy+18y²)
=21x²-20xy-7x+4y²+2y-36x²-51xy-18y²
=-15x²-71xy-14y²-7x+2y
(7x-2y)(3x-2y-1)-(4x+3y)(9x+6y)
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