Answer
$a_1 = 1$, $a_n = a_{n - 1} + 3$
Work Step by Step
In a recursive formula, we need to give the initial condition, $a_1$, which is the first item in the sequence:
$a_1 = 1$
To find the recursive formula of the sequence, we must first find the pattern.
In this exercise, $a_n$ is found by adding $3$ to the previous term, $a_{n - 1}$:
$a_n = a_{n - 1} + 3$