When solving for the coefficient a in the activity, you need to substitute a point on the parabola in for x and y. In the activity, you used one of the roots of the parabola; could you have also used the vertex for x and y? What would happen?
A parabola has a characteristic arc appearance when graphed on a Cartesian plane. The vertex is the center of the arc. It is either the maximum point or the minimum point, depending on the form of the equation. Since the parabola passes through the vertex, then it is still valid to be substituted as a point. You would still come up with the same answer for coefficient a.
