Because triangle ABC is an isosceles triangle, AB = AC = X.

Because the circumference of △ABC is 18, 2X+8= 18 =X=5.

So AB=AC=5

2 because △ABC's outer angle ∠c= 120 and inner angle ∠ C = 180- 120 = 60.

∠ B = ∠ A ∠ C = 60, so △ABC is an equilateral triangle, so ∠ A = ∠ B = ∠ C = 60.

3 Because AD is parallel to BC ∠ B = 50, ∠ Bad = 50.

Because AB bisects the angle ∠DAC, ∠ DAC = 100.

So ∠ a = 50.

So ∠ c = 180-50-50 = 80.