Do you know how to find the midpoint of a line segment drawn between two given points?
Do you know how to find the distance between two points?
Do you understand why a line from a chord to the centre of a circle is perpendicular to the chord?
If you do not understand these things then you do not have the background knowledge necessary to answer the question you posted, in which case you need to go back and review the relevant material.
Edit: All this stuff was addressed nearly two weeks ago in this thread:
http://www.mathhelpforum.com/math-he...tor-chord.html. You're expected to have learned something from all the help you got there.