Solution:
Given the equation below
![x^2-6x=12](https://img.qammunity.org/2023/formulas/mathematics/college/4kkf6hy1dp0sb8s28m88niag5uh5nz1ncy.png)
Applying the completing the square method
Where the general form of a quadratic equation is
![ax^2+bx+c=0](https://img.qammunity.org/2023/formulas/mathematics/high-school/mvkhuzwnjhb4epaf7jjcoq2vi4zdi4350m.png)
For the completing square method,
![Add\text{ }((b)/(2))^2\text{ to both sides of the equation}](https://img.qammunity.org/2023/formulas/mathematics/college/43d3efnorwnyjtdlfw65bn5qfevjuuhjs6.png)
Where
![b=-6](https://img.qammunity.org/2023/formulas/mathematics/high-school/1ftpq2eqxl0z0q21yu04amxapq7utxo1fx.png)
The number that should be added to both sides of the equation to complete the square is
![=((-6)/(2))^2=(-3)^2=9](https://img.qammunity.org/2023/formulas/mathematics/college/dahjjkkf02v99pux5m6uslx9yo609zz5wv.png)
Hence, the number is 9 (option B)