Use (a + b)² = a² + 2ab + b². Here a = x and b = 6, so the middle term is 2 × x × 6 = 12x. When squaring a binomial, remember that the middle term comes from multiplying the two terms together twice. That is why the middle term is 2ab. 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.