What sets of numbers does -17 belong?
belongs to an infinite number of sets.
For example, the Real Numbers, the Rational Numbers, Integers,
negative integers, odd negative integers, negative primes numbers,
the set {12, -17, 98} or {2.76, pi, -17, k, wikianswers}. In fact
any collection, however random, of numbers or other things, that
includes -17.