Answer: x = 10
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The two blue chords are equally distant from the center. This is shown by the segments with single tick marks. These segments are 3.5 units long.
The bottom blue chord is 10 units long because half of it is 5 units.
Since the chords are the same distance away from the center, this means that the blue chords are equal in length. Both blue chords are 10 units long.