Because the distance from the center of the circle to each of those chords is 6, then there's a theorem that says that the chords are also the same length. Because on portion of one of those chords is marked with a single slash mark and measures 9, then the other part of that same segment, also marked with a single slash mark is 9. That makes the whole segment measure 18. Assuming that x is indicative of the whole segment and not just a portion of it, then x = 18
