How do I determine the net realizable value of the following?

The general ledge reported a/r of 2,150,000 in jan 2007
the allowance for doubtful accounts had a credit balance of 40,000
throughout 2007 the company wrote off 140,000 and collected 21,213,600 on account from credit customers. this amount includes a NSF check of 3600
Credit sales for year ending dec 31 2007 were 20,000,000
Of these sales 2% were estimated to eventually become uncollectable.

any help would be great.

In practical terms this type of work is called bridging.

With the help of a SPREDSHEET you would need to identify the beginning balance.
Each movement fully controlled is then entered in the correct place.
If you deal with each item in turn , making sure you contol to totals , by croos casting
you would come to the CLOSING Balances.

You need to develop this over a period of
months.
One of the tricks is not to clean up the items with a ZERO balance.
Maintain the same list over a period of time.
The spreadsheet helps you to carry forward the same positions over a number of months.