Checks and balances protects against tyranny because

In addition to this separation of powers, the framers built a system of checks and balances designed to guard against tyranny by ensuring that no branch would grab too much power.

Explanation:

why was the national assembly formed by the third estate?

the estates-general had not been assembled since 1614, and its deputies drew up long lists of grievances and called for sweeping political and social reforms. the third estate, which had the most representatives, declared itself the national assembly and took an oath to force a new constitution on the king.

he sent troops to protect the students as they entered the school.