From that diagram that is drawn above, C is the midpoint of the line AB:
AC = 17
AB = 5x - 6
Since C is the midpoint of AB;
AB = 2AC = 2CB:
5x - 6 = 2(17)
5x - 6 = 34
5x = 34 + 6
5x = 40
x = 40/5
x = 8
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