We have to find the arithmetic sequence formula.
The first term is 18 and the common difference would be 5.
Replacing in the formula, we have:
an= a1 + (n-1)d
an= 18 + (n-1)5
an= 18 + 5(n-1) (Reorganizing)
The answer is the third option an= 18 + 5(n-1).