Because it was reflected across the x axis, the distance between the two points is twice the distance of the old point from the x axis, and the distance from the old point from the x axis equals the distance from the new point to the x axis. The distance from the x axis is the absolute value of the x=y value. 7.5 is the y value, meaning the point is 7.5 units away from y=0, the x axis. The new point is twice this from the old point. 7.5 x 2 = 15. The new point is 15 units away from the old point.
