Step-by-step explanation:
Firstly take the unknown angle be x
now x + 138° = 180° {: linear pair}
x = 180-138
x = 42
now , angle 1+ angle x + 18 = 180 { because of angle sum property }
=> angle 1 +42 +18 = 180
=> angle 1 +60 = 180
=> angle 1 = 180-60
=> angle 1 = 120
