Alright.
Firstly the formula to find the circumference of a circle is 2 * pi * radius
In this case the circumference is 2*pi
However we want the circumference for an angle of 120 which is 1/3 of 360.
This gives us 2/3 * pi for the arc.
Then to find the whole perimeter we add the length of the radius on both sides giving a value of 4.1