Answer:
n ∈ ℕ . . . (Natural numbers)
Explanation:
You want the domain of the relation s(n) = 7+3n so that it defines the sequence 10, 13, 16, ....
Domain
The domain of a function is the set of "input" values for which the function is defined. Here, the function is intended for use with values of n that are Natural numbers:
domain = ℕ
__
Additional comment
In general, the domain of any function that defines a sequence will be Natural numbers. That is, values of n will be 1, 2, 3, ....
Occasionally, the sequence represents a relation that is of interest for values of n other than positive integer values. In such a case, the domain may be Whole numbers, or Real numbers.