Does anyone have any advice on how to work the following question?

Klondike Pharmaceuticals has several herb extractor evaporators that were purchased four years ago at a price of $20,000 (you can ignore inflation). These machines currently require annual maintenance costs of $2,000. However, the maintenance agreement with the manufacturer expires at the end of two years and Klondike expects annual maintenance to increase to $8,000 per year thereafter. The machines could be sold, as is. for $8,000, but there value will decline to $3,500 at the end of two years. At the end of year 6, the machine's useful life will be over and they will be disposed of as valueless scrap.

Klondike is considering replacing these evaporators with a new machine that will accomplish the same job. The cost of the new machine is $25,000 and the manufacturer offers an 8-year maintenance contract of $1,000 per year. This machine will have an 8-year life and be worthless scrap at the end. Assume both of this equipment (new & old) is depreciated using 7-yr MACRS and has a tax rate of 35%. At a cost of capital of 7%, when should Klondike replace its evaporators?