Without knowing the value in column A to validate, we can only answer in generic terms. First, it's easiest for naming if you have the data in each sheet in a table.
Load each table into Power Query using the "From Table/Range" ribbon choice in the Data menu. Let's say the queries are called tblA and tblB.
You will want to merge tblA with a left join on tblB based on the values in column A.
The new merged query (tblC) will have values where tblA and tblB matched. Delete the rows where this happens... now you will have only rows from tblA that did not match to anything in tblB.
Now append tblC query to tblB query (tblD).
Finally, load tblD to a new worksheet - make queries tblA, tblB, tblC Connection Only as there's no need to load them to the data model.