Question 482922
Factor by grouping 
21x^3 - 15x^2 + 56x-40
----
3x^2(7x-5) + 8(7x-5)
----
= (7x-5)(3x^2+8)
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and this one please... 
x^3 +4x^2-2x-8
----
x^2(x+4)-2(x+4)
----
(x+4)(x^2-2)
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cheers,
Stan H.
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