Answer:
a=21
b=69
c=42
d=138
Explanation:
c is supplementary to d (which is 138), so c=180-138= 42.
angle b is on the edge of the central angle 138, so b is 1/2, therefore, b=69.
angle a is complementary to angle b because any triangle that includes the diameter is a right angle, so angle a=90-69=21.