Answer:
1. realizable value = accounts receivable - allowance for doubtful accounts = $45,700 - $3,000 = $42,700
2. the journal entry used to write off the account is:
Dr Allowance for doubtful accounts 420
Cr Accounts receivable 420
Since both accounts receivable and the allowance account are decreased in the same amount, the net realizable value doesn't change (still is $42,700).