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it will take yo from getting started - through advanced calculations
also see Getting Started in the Forums it will help you get the most out of your posts
first - you have 2 files that are joined on account id
you should be able to just place your account ids on rows and the just Sales on the detail marks card
if you are still experiencing multiple columns it means your data is badly structured and needs to be pivoted Pivot Data from Columns to Rows - Tableau
we can be more helpful and specific if you provide your twbx workbook with at least some sample data
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Can you please provide subset/sample data files to better understand the problem statement?