Complex sentence: With Alexander's encouragement and guidance, Cooke began recording as a solo singer in 1957, and achieved great commercial success both before and after leaving Specialty and joining first Keen and then, in 1960, RCA.
Split into simpler sentences: With Alexander's encouragement and guidance, Cooke began recording as a solo singer in 1957. He achieved great commercial success both before and after leaving Specialty and joining first Keen and then, in 1960, RCA.