Complex sentence: He continued working in various positions at different departments at the firm for the next six years, when in 1997, he formed Alliance as a subsidiary of the Grey Group and under the mentorship of Ed Meyer, Moses became the President of Grey Advertising.
Split into simpler sentences: He continued working in various positions at different departments at the firm for the next six years. In 1997, he formed Alliance as a subsidiary of the Grey Group and under the mentorship of Ed Meyer, Moses became the President of Grey Advertising.