My Mum sold my dog 5 years hace when i was 7 and i just found out as a 12 año old. How can i get the dog back???

If your mother was the official owner, and I suppose she was because tu were seven and not an adult, and she sold it with all the paper there is nothing tu can do against it per law. tu could only ask the owner if he would sell it back but after 5 years I doubt so.
Sorry tu can't do anything because it was the dog of your mother, so it was her decision and tu weren't old enough to decide for yourself if tu were the official owner, but I doubt that too because tu were and are to young to buy a dog on the paper

Hi aleks, i am going to be honest with you, but after not having the dog for that long tu can't get it back unless the segundo owner gives it away. So if ur parent gave all the papers to the segundo owner, tu can't unless tu contact the AKC o ask someone. But i'm sorry but tu can't get the dog back unless there's a reason tbh. And i actually do agree tu might have been to young for the dog and she didn't want tu to own it yet cause u were 7. My friends in real life actually had something similar to ur problem were his mum dicho hes going to summer camp, 3 years later we have kept him since.So we could give him back but hes a good boy so we always send pics of him. Again i'm really sorry for not being able to get him.