1) Considering the equation x²-10x +10 let's rewrite it completing the square
1.1 Let's divide the coefficient of the term 10x by 2. To find the most closer term of the perfect square. As 10 :2 = 5
Then:
(x-5)² =x²-10x +25
2) Since there was added 25 on the left side, we need to add 25 on the right side to keep this identity:
x² -10x +25 +10 = 0 +25
(x-5)² +10=25
(x-5)² =25-10
(x-5)² = 15
3) Since the question does not ask us to solve it. Then we can stop it here.