Use (a − b)² = a² − 2ab + b². The last term is positive because (−6) × (−6) = 36. When squaring a subtraction, the middle term is negative but the last term is positive because a negative times a negative is positive. Expanding the chosen answer is a good check that it returns the original expression. The correct answer is x² - 12x + 36, which matches the calculation or reasoning shown.