Not sure what to put for doubtful debts. Can I please have the answer and working out?

On 1 July 2017, A Ltd had Accounts Receivable and Allowance for Doubtful Debts accounts as below. Ignore GST. * ccounts Receivable 1/7 Balance 21,000 * llowance for Doubtful Debts 1/7 Balance ,000 During the year, the following transactions occurred: 1. Accounts receivable collected $7,000. 2. Accounts written off as uncollectable $7,000. Based on an ageing of accounts receivable on 30 June 2018, Austin, the accountant, determined that the Allowance for Doubtful Debts account should have a credit balance of $15,000 on the balance sheet as at 30 June. Complete the amount xxx in following general journal to adjust the Allowance for Doubtful Debts account. Cr. Allowance for doubtful deb-D