Every parabola turns exactly once, and the turn has coordinates. The x-coordinate is $h = -\dfrac{b}{2a}$ — read aloud: h equals minus b over two a. That vertical line is the axis of symmetry: the graph is its own mirror image across it, which is why h also sits exactly midway between the two roots whenever roots exist.

The height of the turn is $k = c - \dfrac{b^2}{4a}$ — k equals c minus b-squared over four a. With a positive, k is the lowest y the function will ever visit; with a negative, the highest — the vertex is where every max-and-min question in this course eventually goes to be answered. Work h first, k second, and mind the signs: −b with b = −8 comes out a cheerful +8.