Complex sentence: In 1958, he got an assignment that would launch his career as a photographer, when he assembled 57 legendary jazz musicians, for "Esquire magazine" in 1958 in Harlem.
Split into simpler sentences: In 1958, he got an assignment that would launch his career as a photographer. He managed to assemble 57 legendary jazz musicians, for "Esquire magazine" in 1958 in Harlem.