Okay so basically, you subtract 18 on both sides since the 18 doesn't work, making the equation
x² - 10x = -18
So then you add (b/2)², which is 25, to both sides, to get
x² - 10x + 25 = 7
After this, you factor.
(x-5)(x-5) = 7
Then you simplify.
(x-5)² = 7
Then you solve for x.
√(x-5)² = √7
x-5 = (+ or -)√7
~ x= 5 (+ or -)√7 ~
Hope this helps!