How can I convince my parents to let me get a cat?

I am 14 years old and very very mature... My mum likes cats and my dad hates them (or at least says so) I am willing to feed the cat, clean out its litter and play with it every day. My parents used to have cats but one of them refused to use its litter box so they had it put down... We have one dog that I am very helpful with like letting him out and feeding and playing and what not. My dad refuses to listen to my side of the argument....when ever I talk about having a cat he says. "So the cat in your apartment." My dad is the big op-poser of me getting a cat, my mum has even said if my dad didn't oppose cats so much we would pro-bally already have one...