Well, the formula for the length of an arc is;
n⁰ × 2πr
360
Where; n⁰ is the angle
π is pie, which is 22/7 or 3.143 approximately
and r is the radius
Anyway, according to the question;
n⁰ is 120⁰
and r is 5 meters. And as you know, the pie is 22/7. So that will be
120⁰/360 x 2 x 22/7 x 5
And that will give you 10.47metres. Which is approximately 11 metres. Hope i helped, have a nice day