How to convince my mom to let me adopt a dog from the Humane Society?

So, I'm 13, I'm a guy, really responsible and I want to an adopt a dog from the Joplin Humane society, I live next to Joplin Missouri and there are over 900 pets HOMELESS! It makes me sad, and i really want to adopt one, we have a dog and a cat that behave well and i help take care of the dog and the cat is my brother's and i help him take care of it. So any ideas?