The y-coordinate of the midpoint of a line segment is the average of the y-coordinates of the endpoints of the line segment.
This is: call y1 the y-coordinate of one endpoint and y2 the y-coordinate of the other endpoint of the line segment, then the midpoint is [y1 + y2] / 2.
The method, then, is to add the two y-coordinates and divide by 2.
For the vertical line segment with endpoints (0,0) and (0,15) theprocedure is [0 + 15] / 2 = 15/2 = 7.5, and the result is 7.5