Answer:
15
Step-by-step explanation:
Here we are given the x intercepts at x=3 and x=9
So let us try to make quadratic equation for this using given information.
We have factors in the form (x-a)(x-b)
where a and b are x intercepts given to us.
So rewriting in factored form:

Now let us simplify it:

Let us find the vertex now,
For a quadratic equation of the form:

For x coordinate , we have the formula,

So using this formula for our equation,

So x = 6
Answer: The x-coordinate of the parabola's vertex is 6.
The slope is 0 I’m pretty sure because it is a horizontal line, if it was a vertical line it would be undefined