Here are the answers! :)
1. Answer: The political idea of Popular Sovereignty allows individual states to vote on the issue of slavery.
2. Answer: The U.S. Congress ensured balance of free and slave states when Missouri wanted to enter the Union as a slave state in 1820 by admitted the state of Maine as a free state and Missouri as a slave state. This kept the balance.
3. Answer: The Missouri Compromise addressed the issue of the spread of slavery into western territories by prohibiting slavery in the Louisiana Territory north of the 36° 30° latitude line, with the exception of Missouri.