How did The Missouri Compromise keep the balance of power?

Before the Missouri Compromise, there were 11 free states and 11 slave states. Missouri then came in as a slave state and the line of demarcation became 36 degrees 30 minutes north. Maine came in as a free state, which kept the balanced of slave and free states.

The Missouri Compromise

Under the arrangement that became law in 1820, known as the Missouri Compromise, Missouri entered the state as a slave state, and Maine entered as a free state. In addition, the territory that remained from the Louisiana Purchase was divided into along the 36 Degrees 30 Minutes north latitude.

With the exception of Missouri, slavery was prohibited in states north of that line.

The South agreed to the compromise in order to secure an additional slave state. But it emerged from the controversy in a state of shock. For the first time on the floor of Congress, slavery was denounced as a moral evil. It would take another 40 years for the American Civil War to occur, and decide whether the slaves should be freed or not.
Maine