Explanation:
let's have a good look at the sequence :
5, -1, -7, -13, -19, ...
do we see a pattern ?
I do.
look at the differences between the elements of this sequence.
do you see it now ?
every element (except for the first one, of course) is smaller by 6 than the previous element.
that is consistently so throughout the whole visible sequence.
therefore the correct answer is
f(n+1) = f(n) - 6