Case, the original direction is the same, which means parallel. One case is the internal angle, so it is 120, parallel. There are two theorems that two straight lines are parallel and their internal angles are equal. The second angle ∠B is 120.

∫∠A = 120

∵ Two straight lines are parallel with equal internal angles.

∴∠A=∠B

∴∠B= 120

I don't understand. Keep asking.