That would depend on the circumstances under which you determined that M is in fact the midpoint of AB.
Is it a given fact?
Is it a midpoint because the coordinates of M are the average of the corresponding coordinates of A and B?
Were you told that some segment containing M is a bisector of AB?
I can't possibly guess what your figure looks like or what other information you have. So I can't help you, and, given the paucity of information provided, I would wager a goodly sum that no one else can either.
John
My calculator said it, I believe it, that settles it