Anna is a songwriter who collects royalties on her songs whenever they are played in a commercial or a movie. anna will earn $30 every time one of her songs is played in a commercial and she will earn $80 every time one of her songs is played in a movie. anna's songs were played on 3 more commercials than movies, and her total earnings on the royalties from all commercials and movies was $640. write a system of equations that could be used to determine the number of commercials and the number of movies on which anna's songs were played. define the variables that you use to write the system.