How do I come up with the correct calculations to record in a journal entry?

How do I come up with the correct calculations to record in a journal entry? I know how to record the expense – I can’t figure out what number to write. On September 1, 2018, the company borrows $20,000 from a local bank and signs a note. Principal and interest at 12% will be paid on August 31, 2019.

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