How do I convince my parents to let me buy a laptop?

I really need a laptop in order to play the sims 3 and other things. I told my mom that I need to save up my money in order to get one and she basically said no. I feel like she doesn’t think I am responsible enough to buy one but I am a teenager and I am. I need one for homework and other things! I don’t understand why my mom can’t just let me do chores and get an allowance so I could buy something cheap