If I want to represent all the base 10 integers that are multiples of 9 as a language how do I do so?Alphabets are finite sets.
The language of all base-10 integers that are multiples of 9
formal languages;regular languages
You need to check the definitions.  An alphabet is a finite set of characters (or symbols) that are used to write strings; a string is a sequence of characters from some alphabet.When you write numbers down, you write them as a sequence of symbols.  What symbols are those?  That's your alphabet.