How do I do a formula to join two fields together like first name and last name into full name?

Having issues with this in my custom module. In most CRM systems you would do:

FIELD1 & " " & FIELD2

But it gives an error saying I'm not supposed to use double quotations so not sure what that means. I've tried to read the formula help guide but it's not very helpful, most examples are just for numbers and what not and don't give examples for string?

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