Answer:
a. Journal entries to record the reinstatement of the account receivable
Account Title and Description              Debit   Credit
Account receivable account                 $600
    Allowance for Doubtful Accounts account         $600
(Reinstatement of the account receivable)
b. Journal entries to record the receipt of cash
Account Title and Description       Debit   Credit
Bank Account                     $600
    Account receivable account             $600
(Receipt of cash)