Triangle ABC has AB = BC. So it's isosceles triangle. So <A = <C
Given: <B = 80
Sum of interior in a triangle = 180
<A = <C = 1/2(180 - 80)
= 1/2(100)
= 50
<BCA + <BCD = 180 (supplement angles are equal 180)
so
<BCD = 180 - <BCA
<BCD = 180 - 50
<BCD = 130
Answer
<BCD = 130°
