$Why$ is the axis of symmetry of a parabola $-{b\over 2a}$ and ${not}$ ${b\over 2a}$? I'm working on a lesson plan for my students regarding completing the square for a parabola, and I've done the following:

$$\begin{align}ax^2+bx+c &= a\left(x^2+{b\over a}x\right) + c \\ & = a\left(x^2+{b\over a}x+{b^2\over 4a^2}-{b^2\over 4a^2}\right)+c \\& = a\left(x^2+{b\over a}x+{b^2\over4a^2}\right) - a{b^2\over4a^2} + c \tag{$*$}\\ &= a\left(x^2-2hx+h^2\right)-ah^2+c \tag{Let $h=-{b\over 2a}$}\\ &=a(x-h)^2 + c-ah^2 \\ &= a(x-h)^2+k.\tag{Let $k=c-ah^2$}
\end{align}$$

This gives us the standard form of a parabola we all know and love to pick out the vertex of the parabola, but how am I going to justify the selection of $\displaystyle h = -{b\over2a}$ over $\displaystyle h={b\over 2a}$? The selection is to give us the axis of symmetry $\displaystyle x=-{b\over2a}$, of course, but how do I explain why the negative gives us the axis of symmetry? I found a demonstration here, but for my purposes it seems like a chicken-and-egg situation: I haven't discussed the quadratic formula yet, which I will be proving the quadratic formula as part of the lecture, but I need to discuss completing the square first. So it seems kind of silly to use the quadratic formula to prove a result that will be used to prove the quadratic formula.
I know that by definition, the axis of symmetry is the line where any two $x$ values of the quadratic equation $ax^2+bx+c$ have the same $y$ value. But I'm stuck with trying to find these $x$-values otherwise. Do I need to reorganize my lesson plan? 

Regardless of what choices are involved, completing the square leads to
$$ a\left(x + \frac{b}{2a} \right)^2  + \left(c - \frac{b^2}{4a} \right) $$
Since you were trying to arrive at a formula of the form
$$ a (x-h)^2 + k $$
The choice of $h = -\frac{b}{2a}$ is nothing more than pattern matching
$$ x - h = x + \frac{b}{2a} $$
