Factor completely 64 + 27a^(3). Im stuck at 4+3a (16 +9a^(2). the answer is 4+3a(16 - 12 + 9a^(2). I dont understand where the 12 came from.

Question

zepdrix · Answer

Ummm there is a formula for calculating the sum and difference of cubes.
It looks something like this I think:
$$a^3+b^3=(a+b)(a^2-ab+b^2)$$

Your equation starts out as:
$$4^3+(3a)^3$$

See where that extra term is coming from? :)

zepdrix · Answer

|dw:1348979987018:dw|
see why the subtraction is necessary? :D