Receipts from cash sales of $7,500 were recorded incorrectly in the cash receipts journal as $5,700. What entry is required in the depositor's accounts?

So you recorded.

Dr. Cash 5,700
Cr. Sales 5,700

You should have done:
Dr. Cash 7,500
Cr. Sales 7,500

So to fix it, all you need to do is:
Dr. Cash 1,800
Cr. Sales 1,800