Problem with converting numbers from HEX/DEC file to numeric format

I work as an implementation technician for vending machines that dispense PPE (personal protective equipment) in companies. Employees use their RFID cards to collect items by tapping them on the reader installed in the machine.

My problem is that I received card numbers from the client, but they are provided in HEX format (e.g. 000004B00921209) or DEC format (e.g. 3.90845E+11). ChatGPT suggested that they can be converted into the format used by the machine — an 8-digit numeric format — but I’m unable to perform this conversion correctly.

I’ve already tried using Power Query in Excel as well as various online converters, but nothing has worked so far.

I would appreciate any help or guidance on how to properly convert these values.