How do you get the vertex by completing the square in algebra 2?

there's an example in my algebra 2 book g(x)2x^2+12x+13 = 2(x^2+6x+9)+13-2(9) = 2(x+3)^2-5 = 2[x-(-3)]^2+(-5) which all equals (-3,-5) and the equation for the axis of symmetry is x=-3 my question is where did they get the 9??