Hi there!
The x-coordinate of the vertex is the mean, or average of the x-intercepts of the parabola.
Let's take the mean of 3 and 9, which are the x-intercepts.
[tex] \frac{3+9}{2} [/tex]
[tex]= \frac{12}{2} [/tex]
[tex]=6[/tex]
That's your answer.
Have an awesome day! :)
~AgentCozmo4, Junior Moderator