Hiding information, selling faulty equipment and bad customer service

I bought a laptop in October and am a part time student working which means I don’t use the laptop everyday. From the start the computer was slow but it was bearable but recently it has been worse it takes forever even to turn it on. I went to the store to exchange it and get something better and I was told I can’t because I’ve passed 21 days which I was not even aware of. The lady that was selling to me told me about a 6 months period if there’s something wrong. I am so disappointed with their customer service because they’ve hidden information from me. If I knew about the 21 days I would have taken the laptop back but I relaxed because I knew I have 6 months warranty. The computer doesn’t work it just stares at you.