Drive the quad bike out of the shed and turn left.
Drive up the ramp on the side of the next building.
Brake or lightly tap the reverse button when you reach the top to avoid going over the edge.
Dismount and pick up the GENERATOR ACCESS CARD. (A keycard the size of a magazine? Only in Tomb Raider!)
If you accidentally drive off the other side of the roof, you can drive around and try again, or walk to the side of the storage building, climb onto the crate, and then jump to grab the grate on the side of the office. This grate extends only after you drive the bike onto the roof. Climb up and cross the roof to get the card.
Climb or drive down off the roof and go into the office/rec room (the building you were just on top of). As you pass the pool table, a computer technician/pool shark attacks from the right. Take care of him.
Then go into the back office, turn right, and use the GENERATOR ACCESS CARD in the card reader to open the door. You'd think the bearded guy in the cell would be happy to be free, but no. He takes out is pent-up frustrations on Lara. Shoot him, go into his cell, and throw the switch to turn off the electric fence.
Now you can return outside and safely use the switch on the electric fence to open the gate.
Drive the quad bike out, turn left, and head down the wide tunnel.
Drive past the dead snakes and the yellow door where you pulled the switch earlier. Gun it up the stone ramp...
...to jump the fence and end the level.