A recursive formula for a sequence expresses the [tex]n[/tex]-th term as a function of the previous one(s).
In this case, you can see that every term is 6 more than the previous one.
So, the recursive formula is "the [tex]n[/tex]-th term is 6 more than the [tex]n-1[/tex]-th term".
In formula, it's written
[tex]a_n=a_{n-1}+6[/tex]
Since the recursion could go back indefinitely, we usually set a starting point, so that the sequence is fully determined:
[tex]\begin{cases}a_0=20\\a_n=a_{n-1}+6\end{cases}[/tex]
