Final answer:
The Missouri Compromise addressed the issue of slavery in new territories and affected Massachusetts by allowing Maine to enter the Union as a free state and Missouri as a slave state.
Step-by-step explanation:
The Missouri Compromise had a significant impact on Massachusetts. It addressed the issue of slavery in new territories by allowing Maine, which had been part of Massachusetts, to enter the Union as a free state, and Missouri as a slave state. This compromise maintained the balance between free and slave states in the Senate.