You'll have to go back to your records from 2017 and 2018 to figure out what you actually paid for the stocks you received. That is your basis.
During the merger the number of shares you owned probably changed. But what you paid for them didn't. You need to know how many shares you own post-merger as well as what you paid for them.
Then you'll figure out what you paid per share. Then you'll figure out how many shares were sold on the 1099-B you received and enter in what you paid per share for that many shares and that is it.
Save all of these records for if there are any questions and when you sell the remaining shares.
@murlax
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