Answer and Explanation:
The Journal entry is shown below:-
1. Cash Dr, $10,400
              To Sales $10,000
               To Sales taxes payable $400
(Being the cash is recorded)
Here we debited the cash as  it increased the assets and we credited the sales and sales tax payable as  it increased the sales and the liabilities 
2. Cost of goods sold Dr, $5,000
                  To Merchandise inventory $5,000
(Being cost of goods sold is recorded)
Here we debited the cost of goods sold as it increased the expenses and we credited the merchandise inventory as  it reduced the assets 
3. Unearned services revenue Dr, $50,000
               To Earned services revenue $50,000
(Being unearned service revenue is recorded)
Here we debited the unearned service revenue as it decreased the liabilities  and we credited the earned service revenue as it increased the revenue