Find the cardinal number A={9,14,19... 84}

If I tell you, you won't remember it. It's is pretty simple
Google cardinal numbers and learn the definition.

AS noted, Google "Cardinal Numbers". However, here is the answer

Formal definitionFormally, assuming the axiom of choice (https://en.wikipedia.org/wiki/Axiom_of_choice), the cardinality of a set X is the least ordinal α such that there is a bijection between X and α. This definition is known as the von Neumann cardinal assignment (https://en.wikipedia.org/wiki/Von_Neumann_cardinal_assignment)