Question 306307
Without a diagram I assume that you are trying to find the arc length subtended by an angle of 240 degrees of a portion of the circumference of a circle of radius 12. If so, then you can use a proportion to solve this.
It goes like this...240 degrees is to 360 degrees (the entire circle) as arc AB is the the circumference of the circle ({{{2*pi*R}}}). 
{{{240/360 = AB/(2*pi*R)}}}
{{{240(2*pi*R)/360 = AB}}} Substitute R = 12 then evaluate.
{{{highlight(AB = 16*pi)}}}