<PRE><B>Question 960756</B>
7x^2 -21xy -28y^2
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the coefficient of x^2 is 7 which is a prime number, therefore we have
(7x +or- ?) * (x +or- ?)
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now we factor 28 (coefficient of y^2)
1 * 28
2 * 14 
4 * 7
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we want a -21 (coefficient of xy), therefore
(7x + 7) * (x - 4) gives us the desired results
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