Answer:
What journal entry did Pincus record to write off uncollectible accounts during 2021
Dr Allowance for Uncollectible Accounts	$ 6,300
Cr Accounts receivable	$ 6,300
What journal entry did Pincus record to recognize bad debt expense for 2021?
Dr Bad Debt Expense	$ 8,040
Cr Allowance for Uncollectible Accounts	$ 8,040
Explanation:
Pincus provided a total of $156,000 of services on account.  
Dr Accounts receivable	$ 156,000
Cr Sales	$ 156,000
In 2021, the company wrote off uncollectible accounts of $6,300  
Dr Allowance for Uncollectible Accounts	$ 6,300
Cr Accounts receivable	$ 6,300
By the end of 2021, cash collections on accounts receivable totaled $132,300.  
Dr Cash	$ 132,300
Cr Accounts receivable	$ 132,300
Balances on Accounts 31.12.2012 before adjustment  
Accounts receivable CREDIT	$ 17,400
Allowance for Uncollectible Accounts DEBIT	$ 6,300
Pincus estimates that 10% of the accounts receivable balance at 12/31/2021 will prove uncollectible.  
Dr Bad Debt Expense	$ 8,040
Cr Allowance for Uncollectible Accounts	$ 8,040
If the company applies the allowance method, it means that the account Allowance for Uncollectible Accounts must show as balance the % of estimated value.
Because the company already has a DEBIT balance ($6,300) in the Allowance for Doubtful Accounts  it's necessary to register an entry that COMPLEMENT ($8,040) the existing value and reflect the value estimated as bad debts ($1,740).
Bad Debt Expense = $8,040 - $6,300 = $1,740
It's necessary to reflect $1,740 in the Allowance for Uncollectible Accounts as Credit, so we need an entry of $8,040.