Multi-step angle problem with isosceles triangle

Question 5 (Non-calculator, Higher, 4 marks)

Triangle $ABC$ is isosceles with $AB = AC$. Angle $BAC = 40°$. Line $BC$ is extended to $D$.

(a) Find angle $ABC$. Give reasons. (2 marks) (b) Find angle $ACD$ (the exterior angle at $C$). Give a reason. (2 marks)