When you know a term and the common difference/ratio and want to come up with another term or the general formula
Finding the nth term of an arithmetic sequence:
where 
Finding the nth term of a geometric sequence:
where 
𝑎𝑘=term you know
𝑑=common difference 
𝑎𝑛=n-th term
𝑔𝑘=term you know
𝑟=common ratio 
𝑔𝑛=n-th term
So,
So,