What are the explicit equation and domain for an arithmetic sequence with a first term of 5 and a second term of 3?

Any arithmetic sequence can be expressed as:

a (n) = a+d (n-1), a=initial term, d=common difference, n=term number

In this case a=5, and d=-2 so

a (n) = 5-2 (n-1) which can be simplified ...

a (n) = 5-2n+2

a (n) = 7-2n

The domain is restricted to integers from 1 to + oo.