Fire Support

Your Pip-Boy will receive a military broadcast while you’re in the area around Beantown Brewery, Cambridge Police Station, College Square, or near Oberland Station. Listen to the broadcast to start this quest then open the objective's location on the map. Head to the Cambridge Police Station further east along the road. You'll hear some shooting when you're near. Head to the location and assist the soldiers in repelling the ghoul attack.

After killing all the ghouls, loot the bodies and grab the ammo around the area then talk to Paladin Danse. You'll have conversation checks and eventually you'll have the option to help them on their mission. After the conversation, the quest will be complete.