Well, you can't do cell-level calculations in Tableau. That's not how Tableau works. You have to think about columns, not the intersections of columns & rows (or CELLS).
In your example, you need to build a calculation that subtracts the 2018 value from the 2017 value, and Tableau will do that for every row in the view you're looking at. Building that calculation will require you to be able to identify the column in your data set that contains the 2018 results, and the column that contains the 2017 results.
The calculation you create will be a brand new column in your data.
Why are you dividing the result of that calculation by "1"?