The company that owns United Airlines, the world’s largest airline, reportedly has agreed to buy US Airways for $4.3 billion in cash. The deal to buy the world’s 10th largest airline would also include the assumption of $7.3 billion in debt. Boards from both companies have approved the deal, which would create an airline behemoth with 6,500 flights a day and hubs on both coasts, The Wall Street Journal reported on its Web site today, citing sources familiar with the deal.