A video-recording system was purchased 4 years ago at a cost of $43,000. A 5-year recovery period and DDB (Double Declining Balance) depreciation have been used to write off the basis. The system is to be replaced this year with a trade-in value of $6,000. What is the difference between the book value and the trade-in value?

The difference between the book value and the trade-in value is $ .