Using the diagram, we can see that there are three arcs, two with given values (200 and 60) and one with the value of x. Because we know that the arc angles must add up to 360, we can use a simple equation, x = 360 - 200 - 60, which is 100. But there is actually a simpler way to do this. See, the angle of BCD, is 50 degrees, and what you should have learned in your class is that the angle of the arc will be double that of the angle. So x is 100.
