Hello there.
We have for a circle that:
[tex] \ell=\alpha r [/tex]
Where:
[tex] \ell [/tex] is the measure of the arc
[tex] \alpha [/tex] is the measure of the angle (radians)
[tex] r [/tex] is the radius of the circle
In our case, we have [tex] \alpha = 120^\circ = \dfrac{120}{180}\pi = \frac23\pi ~rad [/tex]
We have r = 3 cm, then:
[tex] \ell = (\frac23\pi )(3\ cm)\\ \\ \boxed{\ell = 2\pi~cm} [/tex]
