81
The square (x -a)² is ...
... (x -a)² = x² -2ax +a²
You can see the constant term (a²) is the square of half the x-coefficient ...
... a² = (-2a/2)²
The square of half the x-coefficient in your equation is ...
... (-18/2)² = (-9)² = 81
So, your equation is ...
[tex]x^{2}-18x+\boxed{81}=17+\boxed{81}[/tex]
