Sales Forecast

[RK121]

Hello, I am trying to create Sales Forecast using linear regression, but I am not sure I should change my formula.

I have 2 expressions:

 sum(Sales),

 linest_m(total aggr(if(sum(Sales),sum(Sales)),Month),Month)*Month
+linest_b(total aggr(if(sum(Sales),sum(Sales)),Month),Month)

But in my example I don't have month number, I have combination 'month-year'. I will use 3 years of data.

Trying to use file below for my Forecast:
LOAD [Service Order Status],
[Activity Month/Year],
City,
State,
[Total Sales Count]

Please let me know if you have any ideas how to fix it.