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this.<identifier_name> specifies that the identifier is a field of the class since it belongs to "this", to the instance of the class. That is why often you'll see setter methods that look like so:
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Hint #2, you would precede one of the variables with <something>.number. Please fill in the blank: What is <something>?

You have two variables with the same name, one is a class field and the other a parameter to a method, and you must tell the compiler which is which. You can specify the class field by preceding it...