Under the united states constitution, who becomes president of the united states if the president should die? whatif the vice president should die?

If the President should die, the Vice President would become President. If the President and Vice President should die, the Speaker of the House of Representatives would become President.  

Explanation:

Vice President is named as the first in line of succession. Congress is allowed to provide by law for cases in which neither the President nor Vice President can serve. The current such law governing succession is the Presidential Succession Act of 1947.  As specified in the Presidential Succession Act of 1947, The Speaker of the U S  House of Representatives would be next in line if the Vice President were to also die.