Complex sentence: In 1946, Balanchine and Kirstein founded the Ballet Society (renamed the New York City Ballet in 1948) and Kirstein served as the company's General Director from 1946 to 1989.
Split into simpler sentences: In 1946, Balanchine and Kirstein founded the Ballet Society, renamed the New York City Ballet in 1948. He served as the company's General Director from 1946 to 1989.