1) A set is countable if it is either: 1) finite or 2) infinite and there exists a bijection from the set to the natural numbers.
2) A set has the same cardinality as if there exists a bijection from the set to .
In your example, you should know that is countable. and also , thus it is countable and does not have the same cardinality as .
Basically, you have to 'see' if it is possible to make a mapping from the set to or ..