Complex sentence: Cárdenas' efforts to negotiate with Mexican Eagle, in the managerial control of Royal Dutch/Shell and Standard Oil of New Jersey, were unavailing, and the companies rejected a solution proposed by a presidential commission.
Split into simpler sentences: Cárdenas' efforts to negotiate with Mexican Eagle, under the managerial control of Royal Dutch/Shell and Standard Oil of New Jersey, were not successful. The companies rejected a solution proposed by a presidential commission.