To complete the tree of life tomb in Mission of San Juan in Shadow of the Tomb Raider use a fire arrow to swing the platform towards you.
Jump onto the platform and ride it to the next area. Keep following the path and jump down to a lower area on the left. Go up to the totem pole and release the gas. Shoot the flammable gas to release the platform.
Go back to the upper area and shoot the flammable gas with another fire arrow.
When the platform swings up jump over to it and then to the path ahead. Keep following the path and use the platform to get to the cliffs ahead. Go up and around and release the platform.
Jump over to the released platform and jump off the other side and use the grappling hook to swing to the cliffs ahead. Release the gas. Go back over to the first platform and shoot the flammable gas.
Ride the platform up and jump over and climb up to the path to the right. Follow the path. Shoot the flammable gas and then jump to and climb the cliff ahead.
Cut the platform loose. Keep following the path and release the gas. Jump across the nearby platform to the far platform. Use a fire arrow to raise the platform.
Climb back around to the top area and retrace your steps a little and turn right. Go across the wooden beams to complete the tree of life tomb.