Fill in the blank with a number that makes the trinomial have 2 binomial expressions that are same when factored, in this case 36. And after adding 36 to one side do it also to the other side as well.
X^2 - 12x + 36 = -2 + 36 (X-6)(X-6) = 34
(X-6)^2 = 34
(X-6) = + sq root of 34 X -6 + 6 = + sq. root of 34 + 6