Last one of these, but i have some other ones i dont understand if you will help

C + 45° is the Exterior Angle of the Triangle
The Interior Angle with respect to C + 45° is 180° - C - 45° = 135° - C
We know that Sum of Angles in Triangle = 180°
⇒ C - 30° + C + 135° - C = 180°
⇒ C + 105° = 180°
⇒ C = 180° - 105°
⇒ C = 75°