Returning the Favor is a Minuteman settlement quest in Fallout 4. You must complete it to claim Abernathy Farm as a settlement.
To start this quest travel to Abernathy Farm and talk to Blake Abernathy, who will tell you a story about his daughter’s demise at the hands of some raiders. To add insult to injury, these raiders also stole a locket his daughter wore, and he’ll ask you to recover it from them.
You can find these raiders holed up at USAF Satellite Station Olivia (northeast of Sanctuary), and while this quest is mostly a simple smash and grab job, there are some things to be aware of. The Raiders infesting the interior of of the Satellite Station shouldn’t scale, but the two Raiders that occupy the exterior of of the station may scale - at least the one occupying the satellite tower. In addition you can find Bloatflies to the west and Mole Rats to the southeast, and while the former aren’t an issue, the latter include a specimen that has an explosive attached, making it fairly dangerous if it can get close to you.
Inside you’ll find a Laser Tripwire rigged to a Tesla Arc, which is the sole trap in the area. More threatening are the half dozen or so Raiders, led by a unique one named Ack-Ack. While she’s not leveled, she does carry a Minigun, which can mulch opponents if they leave themselves open long enough for the weapon to spin up. Be wary of her offensive output and give her preferential treatment - it’s worth using a Critical to ensure she falls quickly. Don’t get complacent after defeating Ack-Ack, however, as her fellow Raiders aren’t above taking her Minigun and using it.
After clearing the interior of raiders, search the lower, northeastern end of the installation (in the room where you found Ack-Ack and most of the Raiders) to find a red Toolbox near some locked double doors [Novice]. Inside you’ll find the Locket that the Abernathys want recovered. Pick it up, return to Abernathy Farm and give them back their daughter’s locket to earn their trust, after which Abernathy Farm will join you as a settlement, allowing you to develop it as you please.