Question

A parabola intersects the x-axis at x = 3 and x = 9.
What is the x-coordinate of the parabola's vertex?

Answer 1

The vertex is exactly in the middle of the two intersections with the x axis.
So, it's in the middle of x=3 and x=9.

What's the middle of 3 and 9? You can find this by calculating the average.
(3+9)/2 = 6
So, the vertex is at x = 6.