Answer and Explanation:
The computation is shown below:
a. 
The amount of the adjusting entry for bad debt expense should be
= $45,000 + $5,400 
= $50,400
The journal entry should be  
Bad Debt Expense Dr. 50,400
     To Allowance for Doubtful Accounts Cr. 50,400
(Being the bad debt expense is recorded)
b.    
Accounts Receivable	675,000
Allowance for Doubtful Accounts	45,000
Bad Debt Expense	50,400
c.    
Accounts Receivable	675,000
Less: Allowance for Doubtful Accounts  (45,000)
Net realizable value of accounts receivable	630,000