Both angles are vertical angles. According to the Vertical Angles Theorem, vertical angles are congruent. Therefore: 6x°=120° [Vertical Angles Theorem (vertical angles are Congruent)]. 6x/6=120/6 [Division P.O.E] x=20.
Therefore, if x=20, then: 6(20)=120 [Substitution P.O.E. 120=120 [Symmetric P.O.E]