To answer part (b)
Draw lines from the circle centre "O" to "A", "B" and "C".
Label the lengths |AB| and |AC| and the lengths |OB|, |OA| and |OC|.
Consider the triangles OAB and OAC.
Are they congruent or not ?
Then try to find some internal angles in the triangles,
allowing you to find |AB|