What is the general rule for the arithmetic sequence 15 12 9 6?

Answer 1

There are two ways to say the general rule. They both mean exactly
the same thing, and they both generate the same sequence:

1). Starting with 15, each new term is 3 less than the one before it.

2). The nth term of the sequence is [ 18 - 3n ] or [ 3 times (6 - n) ].