After you regain control, walk down the stairs, with the armored enemy on them. Leave the building and start running towards the marked gate, and kill the enemies in your way Outside of the camp, there is a car which you need to get on.
As you drive, you will have to deal with heavy fire on you and, sooner or later, your vehicle will be hit. In this case, you will have to walk towards your destination. On your way there, try and take enemies' car. There is plenty enough time to reach the airstrip even if you go there on foot.
After you reach the airport, get near the helicopter. In the airstrip area, the buildings in which Riley may be kept, will now be marked. Once you come close to the right one, you will hear Riley crying out loud for help. In my case, he was being kept in the prison right to the left of the helicopter. The building's rooftop (the ladder is on the western wall) is also a good vantage point from which you can take out nearby enemies. Kill all guards below and destroy the helicopter that has been following you for some time now. Inside, kill the armored enemy and enter the area where your brother is being kept.
After the cutscene, kill all of the mercenaries that are blocking the passageway to the helicopter. Once you mount the gunner seat, start killing the approaching enemies, but be careful not to overheat the gun.
After you go airborne, watch out for the enemies equipped with RPGs, who will be standing on rooftops of buildings and on containers. The next stage is to shoot down three enemy helicopters.