Rather then just give you the code like the above one did I'm going to walk you through it here so you can actually have an understanding on where you may have got stuck so here is a video with the code and a full guide.
Step 1: Find the 'Clues' note in the library of Croft manor.
Step 2: Find Lara's Crown on the top floor of the library (will need the master key which is acquired by using the invisible ink note on top of the box the crown is locked in by the fire to reveal it's location).
Step 3: Find the Hieroglyphics sheet in the Library.
Step 4: Find Amelia's Painting's and count the number of red boxes on the painting.
Step 5: Get Amelia's necklace that is beside her bed and a note that's called 'An Unlikely Union' which is in the same room.
Step 6: Return to the safe and start the puzzle solving.
Using the Hieroglyphics sheet we can tell that the the 2 birds on the crown equal 5.
The painting we looked at before had 4 red squares.
Lord & Lady croft got married 4 days prior to her birthday which is the 13th making their anniversary the 9th of October.
This will make the safe code number 549.
Now return to the fireplace after looting the safe and turn it over to find more Hieroglyphics. Head down to the cellar and where you moved the two carts before will be a rack you can move. Interact with the panel and input 142.