What constant should you add to the expression $x^2+6x$ to form a perfect square trinomial?

Solve $x^2-4x-5=0$ by completing the square.

Solve $3x^2+18x+11=0$ by completing the square.

Find the vertex of the parabola $y=-3x^2+12x-7$ by completing the square and state whether it is a maximum or minimum.

Which statement best explains why using the completing the square method is the preferred strategy over using factoring and the quadratic formula?