Complex sentence: The B-58 as a weapons system was replaced by the FB-111A, designed for low-altitude attack, more flexible with the carriage of conventional weapons, and less expensive to produce and maintain.
Split into simpler sentences: The B-58 as a weapons system was replaced by the FB-111A. It was designed for low-altitude attack, to be more flexible with the carriage of conventional weapons, and less expensive to produce and maintain.