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 AD and MN are chords that intersect at point B.
 A circle is shown. Chords A D and M N intersect at point G. The length of A B is 9, the length of B D is x + 1, the length of M B is x minus 1, and the length of B N is 15.
 What is the length of line segment MN?
 4 units
 6 units
 18 units
 24 units