Answer:
b = 318°
Step-by-step explanation:
The central angle formed by two radii in a circle is equal to twice the measure of the inscribed angle formed by the same arc at the circumference.
As angles around a point sum to 360°, the measure of angle b is 360° less twice the inscribed angle 21°.
Therefore:
[tex]b = 360^{\circ}-2(21^{\circ})[/tex]
[tex]b = 360^{\circ}-42^{\circ}[/tex]
[tex]b = 318^{\circ}[/tex]
So, the measure of angle b is:
[tex]\Large\boxed{\boxed{b=318^{\circ}}}[/tex]
