After completing Take it Back!, you'll find yourself in the clinic at the Citadel. Elder Lyons will meet you and tell you about the situation. He then sends you to Scribe Rothchild. Speak to the Scribe and he'll tell you that the Enclave are still communicating, despite Raven Rock being destroyed. Rothchild then sends you to meet Paladin Tristan in the Rockland Car Tunnel, do so and talk to Tristan. Next, you have to assist the Brotherhood in taking out the Enclave, follow Liberty Prime, taking out the Enclave as you go. Eventually, the Enclave will destroy Liberty Prime, talk to Tristan and he'll send you into the nearby building. Fight through the building and you'll find a computer room. Download the data from the mainframe and return to Scribe Rothchild.