Can't get around this one at first I thought the answer was small b
Hint: Let $x$ the unknown angle, then $$2x+a=180^{\circ}$$
The sum of angles in a triangle is equal to $180^{\circ}$. So since $A=a$, and $B=C$, then $$B=C=\frac{180^{\circ}-a}{2}$$
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Hint: Let $x$ the unknown angle, then $$2x+a=180^{\circ}$$