Final answer:
If a president vetoes a law, Congress can override the veto and pass the law with a two-thirds majority vote in both the U.S. Senate and the U.S. House of Representatives.
Step-by-step explanation:
If a president vetoes a law, it can still be passed by Congress through a specific legislative process. According to Section 7 Clause 2 of the U.S. Constitution, if the president vetoes a bill, Congress has the ability to override this veto. To do so, a two-thirds majority vote is required in both the U.S. Senate and the U.S. House of Representatives. If both chambers achieve this supermajority vote, the bill can become law despite the president's veto. This is a critical component of the system of checks and balances designed to prevent any single branch of government from becoming too powerful.