Answer:
x^2 + 18x + 81 complete's the square.
Explanation:
y = [x^2 + 18x + (18/2)^2 ] - (18/2)^2
y = [x^2 + 18x + 81] - 81
y = (x + 9)^2 - 81 This is just to make the completion of the square accurate.
You can't just add something to an expression. You have to balance it out.