Answer:
Opposite angles of a cyclic quadrilateral add to 180°
Angel CBA and angle ADC adds up to 180°.
CBA + ADC = 180
CBA = 180 - ADC
x + CBA = 180
x + 180 - ADC = 180
x - ADC = 0
x = ADC
y + ADC = 180
y + x = 180
