Why we put $750 in retained earning? Why the retained earnings need to decrease $50 I don't understand what is retained earning...

On 29 May, JJ's recorded lawn care services provided during May of $750. All clients were paid in cash. JJ's Lawn Care Service Balance Sheet 29 May 2013 Assets Liabilities and Equity Tools & Equipment $ 2,650 Liabilities: Truck 15,000 Notes Payable $ 13,000 Accounts Receivable 75 Accounts Payable 150 Cash 4,175 Total Liabilities 13,150 Equity: Share capital 8,000 Retained Earnings Total $ 21,900 Total $ 21,900 750 On 31 May, JJ's purchased gasoline for the lawn mower and the truck for $50 cash. JJ's Lawn Care Service Balance Sheet 31 May 2013 Assets Liabilities and Equity Tools & Equipment $ 2,650 Liabilities: Truck 15,000 Notes Payable $ 13,000 Accounts Receivable 75 Accounts Payable Cash 4,125 Total Liabilities 13,150 Equity: Share capital 8,000 Retained Earnings $ 21,850 Total $ 21,850 150 700 Now, let's review how JJ's transactions affected the accounting equation