P= 7x² + 5xy - 9y², Q= -3x² + 4y²-6xy,
R= -4x² + 5y² + xy
then, (Q-2P) + 3(P-R)=?
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Answer:
= (-3x² + 4y²-6xy - 2(7x² + 5xy - 9y²)) + (3(7x² + 5xy - 9y²) - (-4x² + 5y² + xy))
= (-3x² + 4y²-6xy + 2(-7x² - 5xy + 9y²)) + (3(7x² + 5xy - 9y² + 4x² - 5y² - xy))
= -3x² + 4y²-6xy -14x² -10xy + 18y² + 21x² + 15xy - 27x² + 12x² - 15y² - 3xy
= -3x² - 14x² + 21x² - 27x² +12x² +4y² + 18y² - 15y² - 6xy - 10xy + 15xy - 3xy
= - 11x² + 7y² - 4xy
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