Answer:
y = -180 + 3x
Explanation:
Because AB = AC -> ABC is a isosceles triangle -> B = C
Because AB = BD -> ABD is an isosceles triangle -> A = D -> x = D
We have ADB + ADC = 180 -> x + ADC = 180 -> ADC = 180 - x (1)
We have x = D, and BAD + ADB + ABD = 180 -> ABD = 180 - 2x but B = C -> C = 180 - 2x (2)
With (1) and (2) We have DAC + ACD + ADC = 180
-> y + 180 -2x + 180 -x = 180
-> y + 360 - 3x = 180
-> y = -180 + 3x