Explanation:
Pick any term. Subtract off the previous one to find the common difference.
And so on. You only need to pick one of those to show as your steps to your teacher. However, doing all three subtractions is a good way to get practice in seeing how we have an arithmetic sequence. The common difference must be the same each time.
We subtract 7 from each term to get the next term, i.e. we add -7 to each term to get the next one.
