The question states that this point is on segment , with the distance between this point and being the distance between to .
It could be shown (using a pair of similar right triangles) that the ratio applies not only to the (sloped) distance between this point and , but to the vertical distance as well.
The vertical distance between this point and would also be the vertical distance between and .
The vertical distance between and is the difference between their coordinates, .
The vertical distance between this point and would be of the vertical distance between and , .
Since is above , any point on the segment between these two points would also be above . Add the vertical distance between and the requested point to the coordinate of to find the coordinate of the requested point: .
See the attached image. Connect the center of the circle to one endpoint of the chord. That forms a right triangle because the radius is drawn perpendicular to the chord -- that's how the distance from center to chord is measured.
The radius splits the chord into two equal parts, each 15 inches long.
The right triangle has legs 8 and 15, so use the Pythagorean Theorem.