How can I convince my mom to let me get a puppy?

Basically, my whole family wants to get a petit basset griffon vendeen because they are adorable and have great personalities. I want a puppy and my mom wants an adult dog. I would be perfectly happy to get an adult dog if I were helping it in some way, for instance if the dog were from a shelter, but you don't generally find a real petit basset griffon vendeen abandoned in a shelter, so we would get it from a breeder. If you're getting it from a breeder, why not get a bouncy adorable puppy right? No, not right. You see, I already have a dog, we got him as a puppy and he was very difficult in every way (although we love him to bits!) and my mom didn't have a lot of help with him because I was little. Now I'm older and I feed, walk and brush my dog, I'm also willing to brush dog teeth although the groomer does that. I'm also relatively knowledgeable on puppy health and I know how to handle puppies so they grow into friendly, loving dogs, so I don't see what the problem is. Does anyone know how I can convince her to let me get a puppy?