The vertex of a parabola is at (12,-4), and its axis of symmetry is vertical. One of the x-intercepts is at (15,0). What is the x-coordinate of the other x-intercept?

distance from one x-intercept from the vertex is the same as the other x-intercept.

the distance from vertex's x coordinate 12 to 15 is 3 thus 3 in the other direction is (9,0)