Passage: In 1966, Tony married the family's housekeeper Joan Gustafson. They had two children, Jennifer and Mario. Madonna resented her father for getting remarried, and began rebelling against him, which strained their relationship for many years afterward. She attended St. Frederick's and St. Andrew's Catholic Elementary Schools, and West Middle School. Madonna was known for her high grade point average, and achieved notoriety for her unconventional behavior. She would perform cartwheels and handstands in the hallways between classes, dangle by her knees from the monkey bars during recess, and pull up her skirt during class—all so that the boys could see her underwear.Madonna's father put her in classical piano lessons, but she later convinced him to allow her to take ballet lessons. Christopher Flynn, her ballet teacher, persuaded her to pursue a career in dance. She later attended Rochester Adams High School where she became a straight-A student and a member of the cheerleading squad. After graduating, she received a dance scholarship to the University of Michigan and studied over the summer at the American Dance Festival.At the age of 19, Madonna began studying dance under the tutelage of Martha Graham, the noted American dancer and choreographer. It was Graham who gave her the nickname "Madame X"; she titled her 2019 album Madame X in reference to this.In 1978, Madonna dropped out of college and relocated to New York City. She had little money while working as a waitress at Dunkin' Donuts and with modern dance troupes, taking classes at the Alvin Ailey American Dance Theater and eventually performing with Pearl Lang Dance Theater. Madonna said of her move to New York, "It was the first time I'd ever taken a plane, the first time I'd ever gotten a taxi cab. I came here with $35 in my pocket. It was the bravest thing I'd ever done." She started to work as a backup dancer for other established artists. One night, while returning from a rehearsal, a pair of men held her at knifepoint and forced her to perform...

Question: What profession did she begin at this point?
backup dancer