If the radius is told, you can simply use πd to find the circumference:
π x (2 x 10) ≈62.8318
This is a full 360 degree circumference measurement however, the easy way to find the length of a specific area is to divide the number by 360 and multiply by the angle of the arc DE:
(31.4159 ÷ 360) x 36 = 6.281 ≈ 6.3 rounded to 1.d.p