You will report the sale as it is shown on the Form 1099-B or consolidated statement that Robinhood sends you for the year of the sale. I'm sure Robinhood will keep track of all the necessary information so that the sale is reported properly.
You should have received a document from the company (or from Robinhood) advising you of the merger and any related tax effects. It's possible that there was no tax effect, and the sale will simply be reported as a sale of B with a basis of $1,000, and with the purchase date being the date that you originally bought A.