Complex sentence: By 1915, the "Montreal Star" dominated the city's English-language evening newspaper market and Graham was able to out-perform his competitors who closed and assured him control of the English-language market.
Split into simpler sentences: By 1915, the "Montreal Star" dominated the English-language evening newspaper market in Montreal. Hugh Graham was able to run his newspaper's competitors out of business, thus assuring control of the English-language market.