(Note to all wikipedians: this is a list of famous Mexicans. If there's anyone missing that a contributor might know about, please feel free to add that name to the list.
... the left of "=", that will make it a perfect square trinomial of the form x2 + 2xy + y2. Since "2xy" in this case is (b/a)x, we must have y = b/(2a), so we add the square ...