Step 1: Find angle a.
Angle a (∠BAC) is an angle at the circumference subtended by arc BC. The angle at the center subtended by the same arc BC is 120∘.
The angle at the circumference is half the angle at the center.
a=21×120∘
a=60∘
Step 2: Find angle c.
Angle c (∠BDC) is also an angle at the circumference subtended by the same arc BC. Angles subtended by the same arc in the same segment are equal.
Therefore, c=a.
c=60∘
Step