From the top wooden ledge, drop to the one below.

Turn left. Jump to the wooden beam and move to the right around the corner. If you hang from the beam, you will traverse much quicker. Jump to grab the ledge and pull up.

Pick up the Treasure (3/13) and kill the spiders that attack. Proceed to the other side of the ledge.

Position Lara to the right side of the ledge because this spot is closer to the horizontal pole. If you jump from the left side, Lara will not reach the pole and you will fall down. You can then climb up the scaffolding and try again. Jump to grab the pole and traverse to the left (close to the wall). Swing and jump to the opposite side. Go around the corner to the left and pick up the Treasure (4/13).