64 is a perfect square, meaning it has two factors that are identical (8), and 36 is also a perfect square, but in this case, you don't see two 6's in the answer choices.
12 and -3 give you a product of -36, so your answer choices would be:
(8x - 3) (8x + 12)
If you're unsure, you can multiply them together to check if they come out to the unfactorized expression.
