Which methods could you use to calculate the y-coordinate of the midpoint of a vertical line segment with endpoints at (0, 0) and (0, 15)?

The midpoint of a line segment is the point that is located exactly in the middle of two points. It is the average of both points, which is the average of the two coordinates x, and of the two coordinates y.
 We have then:
 M = (((x1 + x2) / 2), ((y1 + y2) / 2))
 Substituting values:
 M = (((0 + 0) / 2), ((0 + 15) / 2))
 M = (0, 7.5)
 
 The y-coordinate of the midpoint of a vertical line segment with endpoints at (0, 0) and (0, 15) is:
 y = 7.5

The answer of the above question is

The first one is correct