FOR AN ARITHMETIC SEQUENCE:
"a" is the first term
"n" is the term number (eg. if you're looking for the 10th term, n = 10)
"d" is the difference between the terms
In you case,
d = -2
a = -2
So you would plug it in the formula accordingly,
Then use the distributive property,
And that's the recursive rule for your sequence.
NOTE: the formula given is only for arithmetic sequences, which is a sequence where the difference between the consecutive terms is constant.