, , ,
This is an arithmetic sequence since there is a common difference between each term. In this case, adding to the previous term in the sequence gives the next term. In other words, .
This is the formula of an arithmetic sequence.
Substitute in the values of and .
Simplify each term.
Apply the distributive property.
Multiply by .
Subtract from .
Identify the Sequence 3 , 9 , 15 , 21