x\(^2\) - 18x to be a perfect square.

\((\frac{b}{2})^2\) is added to ax\(^2\) + bx + c in order to make it a perfect square.

\(x^2 - 18x + (\frac{-18}{2})^2\)

= \(x^2 - 18x + 81\)