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mathhelper (a + 2b)(a^2 - 2ab - 4b^2)
don't know what you want done with it.
if expanded you get:
a^3 - 2a^2 b - 4a b^2 + 2a^2 b - 4a b^2 - 8b^3
= a^3 - 8a b^2 - 8b^3