The distance between A and B and the distance between C and D are the same ( the 2 little lines indicate that).
This means the two arc lengths would also be the same.
Set the equations equal to each other,, solve for x then solve for the arc length.
AB = 3x +9
CD = 11x-71
3x+9 = 11x-71
Subtract 3x from both sides:
9 = 8x-71
Add 71 to both sides:
80 = 8x
Divide both sides by 8:
x = 10
Now solve for AB:
3x +9 = 3(10) +9 = 30+9 = 39
The answer is AB = 39