After going through Gen’s Psync in AI: Somnium files – nirvanA initiative, you finally get the third piece of paper containing the information you need to understand Tearer’s instructions. I’ll explain how to do it first, in case you want to reason it out with a little help. Then I will tell you directly what to put in the text box. I’ll start by saying that it’s almost exactly the same as a puzzle you’ve done before, namely the one in the park with the Alfree machine. This is what the pieces of paper say:
2 ONE AR
3 in DG
Thinking back to the Alfree puzzle, you had to look at the pieces and connect them diagonally. You need to do the exact same thing here. Start with HO, then look at the letters diagonally. Then connect these letters to their diagonal. Continue as if IN DG is above HO ST. Matching them correctly will yield HORA INST UNDG, which you will need to enter into the text box as HORAINSTUNDG. It stands for Horadori Institute underground and gives Mizuki information about something the game told us roughly during the Ryuki route.
Once you enter it, you will then need to enter and find the basement. There’s also a puzzle, but the game will pretty much tell you exactly what it is if you backtrack a few times. I guess the testers had trouble reproducing the musical notes. Certainly, as long as you remember the Alfree puzzle, you should have no problem reusing the intersecting arrows to get to the right conclusion. Unless you end up with a practical case of the protagonist’s amnesia, at least. But that then opens up an entirely separate Pandora’s box. And nobody likes worms.