What is the journal entry for this under the periodic inventory system:

Sold goods on account for $21,000 and made cash sales of $18,000.

This is what I think it is. Is this correct or is there more to it?

Dr. Cash 18,800
Dr. A/R 21,000
Cr. Sales 39,800

Making Cash Sales of 18,000

Dr. Cash 18,000
Cr. Sales 18,000

Making AR Sale of 21,000
Dr. AR 21,000
Cr. Sales 21,000

Or,
Dr. Cash 18,000
Dr. AR 21,000
Cr. Sales 39,000

How did you get 18,800 for cash?

Sorry, it was a typo, the amount in the question was supposed to be $18,800 not $18,000.
Thank you for all of your help! You have helped me a lot!

You're welcome.

Then if it was for 18,800, then your answer is correct.