This quest was bugged for me, so don't waste your time until it's fixed. In the upper area as described in the quest log, there are only 2 places to put the bombs. Whenever I'd try to show a hint arrow for the 3rd bomb placement, it would just direct me to the 2nd bomb I placed. Ran around the room, checked everywhere downstairs and can't find where to place this final bomb. Note: If you abandon this quest, there doesn't seem to be a way to get the quest back either. that I'm aware.