Explanation:
Angle ACO = 140° - x. (Sum of angles in a triangle is 180°)
Now draw the line OD. The triangles ODC and OBC are congruent via the SSS congruence property. (OD and OB are both the radius of semicircle, common side OC, and side CD = CB is given)
Hence Angle BCO = Angle ACO = 140° - x.
Since triangle OBC is an isosceles triangle with OC = OB, this means that Angle CBO is also 140° - x.
By Exterior Angle theorem,
we have Angle AOC = Angles BCO + CBO
=> x = (140° - x) + (140° - x)
=> 3x = 280°
=> x = 280°/3. (B)