The dashed line along with each of these parabolas is called the axis of symmetry.  Every parabola contains an axis of symmetry and, as the graph illustrates, the graph to either side of the axis of symmetry is a mirror image of the other side. It means that if we know point onto one side of the parabola we will also know about a point on the other side depend on the axis of symmetry.  We will see how to determine this point once we get into some examples.