I got to the part where you play the mini video game, but I don't know where to get the cartridge. I can't find it in the walkthrough :( I may be dumb
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on either the day where you meet mya at the gas station or the day you go to the hospital, you can take a detour and go to the other floors of the apartment (before you meet mya or before you leave the apartment to go to the pharmacy). the cartridge is in one of the apartment rooms on one of the apartment floors.