An arithmetic sequence is a sequence of numbers such that the difference of any two successive members of the sequence is a constant called the common difference
1) The d represents your difference of the equation. You can find it by finding a pattern between the three numbers given, which is -5
2) Now that we've found our difference, we can plug it into this lovely sequence we have (by the way, your n represents your term that you plug into the equation)
3) This also means that is our first term, which is 2
4) Simplify -->
5)
The formula is confusing at first, but it becomes easier once your learn the patterns!