John K. answered 09/29/14
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Effective Specialist in Never-Been-Good-at-Math situations
This seems like a tricky question the way that it's stated but...
The midpoint of a segment is found by taking the average of the two x-coordinates and the average of the two y-coordinates.
Since the midpoint of the segment is ((a+b)/2, (c+d)/2) and the coordinates of one endpoint are (b,d), the other endpoint must be (a,c).