Query about Question 1 from "PrepSwift: Completing the Square" [3357]

GRE GRE Quant Problem Solving
1

https://www.gregmat.com/quizzes/quiz/prepswift-completing-the-square (Question 1)

Hi greg,

I wanna know why we add -9 in the equation, isn’t that should be +9

2

Where did you get +9? Share your steps.