Which value must be added to the expression x squared plus 12 x to make it a perfect-square trinomial?

To complete the square for x² + 12x, take half of the coefficient of x and square it: (12 ÷ 2)² = 6² = 36. Adding 36 gives x² + 12x + 36 = (x + 6)², a perfect-square trinomial. The correct choice is 36.

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