Solution :
Given :
ABCD is a quadrilateral.
Angle ADC = 90 degree
Angle DCB = 120 degree
Now angle DAB = 70 degree (alternate angles)
We know that the interior angles of a quadrilateral is 360 degrees.
So,
∠ DAB + ∠ ABC + ∠ BCD + ∠ CDA = 360°
70° + ∠ ABC + 120° + 90 ° = 360°
∠ ABC = 360° - (70° + 120° + 90°)
∠ ABC = 360° - 280°
∠ ABC = 80°
Now ∠ x = 180° - ∠ ABC (Line AB extended is 180° )
= 180° - 80°
= 100°
Hence proved.
