Which value must be added to the expression x2 + 12x to make it a perfect-square trinomial? 6 36 72 144

"The correct answer is 36.
A perfect square trinomial can be factored into this form: (x+a)^2Note that (x+a)^2 = x^2 + (2a)x + a^2To find the missing number, we should take the 12 (in the 12x term) and divide it by two to get 6. This number squared will be the missing number at the end of the trinomial.
Let's check that this is the correct answer.(x+6)^2 = x^2 + 12x + 36Indeed, 36 is the correct answer."