Finding the slope of a trendline without a graph
 

Hi,
I have a large amount of data, it is a list of 2000 customer names with
each of their monthly sales for 12 months. I am trying to find an easy way of
ranking these customers based on their sales trend. My thought is adding a
column which shows the slope of their linear trendline. I can get the slope
if I use a graph but I am really just looking for the number. Can someone
help figure out how I can do this using Excel formulas? Thanks for your help.

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Thank You,
Steve