How do I use Excel to calculate a trend (x, y coordinates)?

How do I use Excel to calculate the missing value in the data table below indicated by question marks?

Date:               4/1/2005                                  9/15/2009                                          7/31/2013
Amount:             $22,918.80                                   $13,000.00                                              ???????
Comments:      Purchased New       Appraised by Otts Farm Equipment                        Excel
bobbatAsked:
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Danny ChildIT ManagerCommented:
As I understand it, the items concerned have lost a fixed price per day since their purchase, and you want to extend this trend to the new date.

So, for the loss per day, it's just
(PurchasePrice - 1stAppraisalPrice) / (1stAppraisalDate - PurchaseDate)

And therefore the loss at the 2nd date is
LossPerDay * (2ndAppraisalDate - PurchaseDate)

and the overall remaining value is
PurchasePrice - LossToDate

The attached table does all this in one hit, and the benefit of making it a table is that it makes the (admittedly complex) formula a bit more readable.

I added the 2nd row as a simple test of the maths.
If an item loses 200 in 10 days, it should lose 600 in 30 days... which it does.
note, with the dates, to get a 10 day gap, you have to specify the 1st and the 11th of the month, not the 10th...

Hope this is what you're after!
cheers, Danny
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Danny ChildIT ManagerCommented:
This should do it... I'll explain below.
EE---linear-trend.xlsx
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