Question

How is the -5 gotten in this example of how to complete the square of x^2 – 18x = -86?
x^2 – 18x = -86
x^2 – 18x + (-9)^2 = -86 + (-9)^2
Here’s where I get confused. How is the -5 gotten?
(x – 9)^2 = -5
(x – 9) = ± √-5
Solution:
x – 9 = i √5 and x – 9 = -i √5