Alright remember every quadratic equation can be put in the for ax^2 +bx +c = 0 will I be referring to a,b,and c in this explanation. To solve by completing the square we to move the c to the other side which your problem already has done. We then need to take your b term whick is -18 and divide it by two and square it. We will do that step every time when completing the square so just remember it (b/2)^2. We get 81. We add 81 to both sides. Now we have x^2 -18x +81 = -51 +81 we can simplify this to (x-9)^2 = -30 Most students get confused on how I obtained (x - 9) ^2. The best way I can explain it is when completing the square, out objective is to get a perfect square binomial. So what ever number you got for b/2 will be in your perfect square binomial. Moving on to solve from here we need to FIRST undo the square by taking the square root of both sides and then adding the nine. So our final answer is the positive and negative value of the square root of 30 plus 9. I show step by step how to solve problems just like this on my youtube blog at www.youtube.com/mrbrianmclogan