Investigate the Royal Scribe’s Office
All Story Quests
All Assassin’s Creed: Origins Guides

To reach Chenzira and find the ancient tablet inside Siwa’s Mountain of the Dead in Hideaway in Assassin’s Creed: Origins hit the wall at the back left of the tomb:

To reach Chanzira hit the crack in the wall at the back of the tomb.

Go through the room ahead and turn right at the top of the slope. Go up the slope and turn left. In the room ahead destroy the pots behind the rack on the left wall:

To reach Chanzira and find the ancient tablet hit the pots behind the rack.

Then push the rack backwards and go through the opening. Drop down and Chenzira is standing in front of the ancient tablet.