Texaschic nailed it when saying that it's arithmetic
The recursive way to write this is to say
\(a_1 = 6\)
\(a_n = a_{n-1} - 5\)
which tells us "start at 6 and each time subtract off 5"
The \(a_n\) portion is the nth term while \(a_{n-1}\) is the term just before the nth term
