Because the figure is a semicircle, the circle is cut in half. So, we can find the arc length from the circumference formula and divide that value in half. The circumference is the total length around the circle, so half that length is the arc of the semicircle.
C=2(3.14)r
C=2(3.14)(12/2)
Remember, the radius is half the diameter
C=2(3.14)(6)
C=37.68
Now divide the circumference in half:
37.68/2 = 18.84
Therefore, the arc length is 18.84.