Final answer:
When Missouri was admitted as a slave state under the Missouri Compromise, Maine was also admitted as a free state to keep the balance between slave and free states.
Step-by-step explanation:
Under the Missouri Compromise, when Missouri was admitted as a slave state, Maine was admitted as a free state to maintain the balance between slave and free states in the Union. The Compromise, led by Henry Clay, also established the 36° 30′ line as a boundary: north of this line slavery was banned in the remainder of the Louisiana Purchase territory, except for in Missouri. This legislative action was an attempt to pacify the growing conflicts between pro-slavery and anti-slavery factions and maintain a balance of power.