I assume you want to put this equation of a parabola in vertex form by completing the square.

First, factor out the 2 from the x terms:

Next, take half the coefficient of x,

, square it,

, and add it to complete the perfect square trinomial in parentheses. But, you really added twice that amount since we have a factor of

outside the parentheses. So, to keep things balanced in the equation, we must subtract twice that amount,

, outside the parentheses. 'What one addeth to just one side of an equation, one must taketh away'.