Complex sentence: Returning to St. Petersburg, she opened a studio there in 1894; her subjects included Nikolai Rimsky-Korsakov, Mathilde Kschessinska, Vera Komissarzhevskaya, and other artists, writers, and actors of the time.
Split into simpler sentences: Returning to St. Petersburg, she opened a studio there in 1894. Mrozovskaya's subjects included Nikolai Rimsky-Korsakov, Mathilde Kschessinska, Vera Komissarzhevskaya, and other artists, writers, and actors of the time.