I put your dates in A1:A6 and you numbers in B1:b6. Made a chart and got the
same trendline as you have

Click on the cell displaying Feb-08 and look in the Formula Bar. You will
see a date 01/02/2008 (or 02/01/2008 if you are using US dates - unlikely)
If you format the Feb-08 cell as General it will display 39497.Excel stores
dates as serials number - see http://www.cpearson.com/excel/datetime.htm

So the X you need is the serial date for 1-Aug-08. In a cell type Aug-08
(or in your language Ago-08), format the cell as General and you will see
39661

In A10, I used the formula =SLOPE(B1:B6,A1:A6) to get the slope of 30.21696
In B10 I used the formula =INTERCEPT(B1:B6,A1:A6) to get the
intercept -1187873 ( a more precise version of -1E06)
In A1 I entered Aug-08 and in B11 I computed the forecasted value using
=A10*A11+B10; this gave me 10562

best wishes
--
Bernard Liengme
Microsoft Excel MVP
http://people.stfx.ca/bliengme

"juanpablo" wrote in message
...
I have the following data:

Feb-08 Mar-08 Abr-08 May-08 Jun-08 Jul-08
5000 3802 10400 7600 7400 9800

Whith the following trendline obtained from the chart:

y = 30,217x - 1E+06

How Do I estimate Ago-08
to replace the value of X should be x=9?
What happens for next Feb-09, should x= 14?

Thanks!

JP