When we are completing the square, we are going to move the value of c across the equals. We will do that by adding, and end up with
18=9(x²+6x)
We take the value of b (the coefficient of x), divide it by 2 and square it: (6/2)²=3²=9
This is the value that completes the square. However, since the entire square is multiplied by 9, this value must be multiplied by 9 before we can add it across the equals: 18+9(9) = 9(x²+6x+9) 18+81=9(x²+6x+9) 99=9(x²+6x+9)
When you're out shopping and something you want to buy is on sale. You can take the original price and give it's percentage to find out how much it really is.