For chord CD,
∠CBD = ∠CAD (Angles in the same segment)
∠CAD = 70°
∠BAD = ∠BAC + ∠CAD = 30° + 70° = 100°
∠BCD + ∠BAD = 180° (Opposite angles of a cyclic quadrilateral)
∠BCD + 100° = 180°
∠BCD = 80°
In ΔABC,
AB = BC (Given)
∠BCA = ∠CAB (Angles opposite to equal sides of a triangle)
∠BCA = 30°
We have, ∠BCD = 80°
∠BCA + ∠ACD = 80°
30° + ∠ACD = 80°
∠ACD = 50°
∠ECD = 50°