When you complete the square your quadratic equation must be written in the standard form which is ax^2+bx+c.
First you are going to want to move your c term to the right (in this case move -18 to the right by adding 18 to both sides). This will get you x^2-4x=18. Now you are going to need to complete the square. To complete the square you divide the number in the bx term (in this case -4) by 2 and then square it. (-2^2=4 since a negative times a negative equals a positive). Now that you have 4 you are going to add it to both sides to get x2-4x+4=18+4.
