Answer:
Before 1820, the proportion of slave and free states in US Congress was 11:11. All of this changed in 1820 when Missouri asked to join the Union as a slave state. Thus, the Missouri Compromise was been reached to address the problem to regain balance.
Slavery was prohibited in the area of the Louisiana Purchase north of the 36° 30' parallel (Mason-Dixon Line). In addition, it was stated that new states to join the Union would always be recruited in pairs: one of them should be slave and the other should not. Thus, Maine became a member of the United States that year as a free state.