The previous slide shows an opening provision (b/f) is a doubtful receivable of $200 from a customer called Jones. During the year, we actually recovered this debt (Cr Bad Debt Expense - reversing the expense which would have hit the Profit & loss account the prior year). We also Dr Cash (the $200 received) & Cr Receivables Control account as usual.
Two other debts became doubtful during the year - from customers Adams & Smith. The $450 will have been Debited to the Bad Debt Expense account. The final Bad Debt expense that will be deducted in the Profit & Loss account is $250 (the $450 less the $200 cash received from Jones - the change in the provision). The c/f balance of $450 will be offset against total receivables in the Balance Sheet.
Note that provisions can be specific (as above) or general. That is, a provision of, say, 5% of total receivables if that is a typical amount (usually applicable where there are many small customers).