This is the first puzzle that you have to encounter in Uncharted 4: A Thief’s End. It is not very hard if you are already familiar with Roman Numerals. You just need a keen eye and also know your basic mathematics skills.
If you look at Nathan Drake’s journal, you will see that the arrow symbol has an “X” symbol next to it. The other double M symbol has a “II” next to it. Bear in mind, you are not finding two stones with X and II on them. This is the mistake I did the first time around.
If you know your Roman Numerals, X stands for 10 and II stands for the number 2. If you add up 10 plus 2, you will get the number 12. With this in mind, X plus II should look like XII. XII is the Roman Numeral form of the number 12. All you have to do to open up the door is find a rock with the number XII on it.
It shouldn’t be too hard for you to find the XII symbol in Uncharted 4: A Thief’s End. The triangle button usually prompts you for the areas of interest. You will find it eventually. The screenshot provided above will show you the exact rock you should be look for.