Answer and Explanation:
a. The required balance of allowance for doubtful debts is shown below:
Particulars       Account receivable  %             Estimated uncollectible 
Not yet due    $860,000                1.20%        $10,320
1 to 30 days    $344,000                1.95%        $6,708
31 to 60 days  $68,800                  6.45%       $4,438
61 to 90 days  $34,400                  32.50%    $11,180
Over 90 days $13,760                    67.00%    $9,219
Estimated balance                                           $41,865
b. The adjusting entry is 
Bad debt expense Dr ($41,865 - $14,000) $27,865
          To Allowance for doubtful debts $27,865
(being the bad debt expense is recorded)
For recording this we debited the bad debt expense as it increased the expenses and credited the allowance for doubtful debts as it decreased the assets