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 to get .
Simplify the expression.
Remove unnecessary parentheses.
Subtract from to get .
Add and to get .