Complex sentence: When Feiffer was 17, he became assistant to cartoonist Will Eisner in the mid- 1940s, where he helped Eisner write and illustrate his comic strips, including "The Spirit".
Split into simpler sentences: When Feiffer was 17, he became assistant to cartoonist Will Eisner in the mid- 1940s. There he helped Eisner write and illustrate his comic strips, including "The Spirit".