This is rather a hard concept at first so let's see if we can work through this together.
First of all the provision for doubtful debts is saying 'hey, we don't think we will get this money so we need to account for this'. To do this we need to make a 'provision' that holds these amounts against our accounts receivable ledger as such. In the case of your assessment you have a previous balance of $2500 and you know that a percentage of your Accounts Receivable from the trial balance may not be recovered - the first thing will be to calculate what this new 'irrecoverable' amount will be - so what next?
This new % is the new closing balance that should in fact be showing on your trial balance and not the $2500 that is currently there - this is what is called an adjusting entry - so effectively what is the new total of your provision? Then think about how do I get from the current balance of $2500 credit to the 'new' outstanding credit balance? THIS figure is what your journal will reflect to take you from the current $2500 to the new figure - your journal is NOT the new balance. Journals 'tell the story' of how you got to the closing balances which are showing on your trial balance so to journal the whole amount of the new % you have calculated will inflate your provision for doubtful debts incorrectly.
The next step is to then think, right, I now know how much the 'new' provision for doubtful debts will be (based on the aforementioned % calculation from your additional information) and so I now need to make a new journal that will:
1) Adjust the provision that I have.
2) Adjust the other side of this journal - which will affect your bad debts.
All journals must balance and must have a debit and a credit - so you need to think about what other account is affected here. When a debt goes 'bad' what is the expense account that is affected? This is the other side of your journal in this instance.
You now have the total for this journal - to get you FROM the $2500 to the new provision based on your % calculation and to account for the adjustment to the other side of this journal.