Answer is choice C. We distribute the outer 2 into each of the inner terms (x and -4). Choice D is a trick answer because a lot of the time you might forget to multiply the 2 by the -4 inside the parenthesis (common mistake).
2 times x = 2x
2 times -4 = -8
2 times (x-4) = 2x-8
Side Note: Another path to solve this equation is to divide both sides by 2, then to add 4 to both sides.
