First, I would recommend that you speak to your teacher about her. Specifically mention that she says she can't see the board. The teacher will have access to possibilities like speaking to her parents and connecting them with social services that can help pay for glasses if they can't afford them. I know I didn't want to tell my mom I couldn't see the board because I knew we didn't have much money and that glasses were expensive. Turns out the school and my mom were able to resolve it and I got my glasses. Telling the teacher is a kind thing to do, because it will help her get glasses if she needs them.
This could also have the effect of the teacher (temporarily or otherwise) moving her to the front of the room so she can see better until she gets her glasses and then she can't disturb you any more.
If she calls you offensive names, that can be hard to deal with when you're still in school. Depending on her motivation, it can be next to impossible for you to make her stop. You can try just telling her something along the lines of "That's not a nice thing to say. Don't call me that". Placing a clear, unambiguous boundary sometimes helps with people who are just a bit clueless but not malicious. However, with someone who is bullying you, that is unlikely to stop her. If she refuses to stop when you lay down a clear boundary, your only option is to avoid her and speak to an adult who can help such as a teacher or school counselor. There is unfortunately no interpersonal skill that can stop bullying.
If she stops calling you mean names but persists in asking you to do her homework, then what I've found most useful is to suggest that you study together. For example, if she asks you to do her math homework, tell her "I'm actually planning on working on that in the library after history class. You're welcome to join me." Repeat as needed. Some people who are genuinely struggling will gladly come and do their homework in the same place as you, maybe asking an occasional question but mostly working on their own. Someone who just wants their homework done for them will not show, or come once and never again when they realise that you won't do their work for them. If she shows and then keeps asking you to do her work completely for her, then respond "I'm happy to answer specific questions, but I'm not doing it for you." Again repeat as needed. Here, something like 'Show me how to solve question 2" is too broad and gets the canned response above, but a question like "I get a negative length as an answer on question 2, do you know what I'm doing wrong?" is a question you can help with.
If she wants to study with you all the time, then you can lay down a clear boundary that works for you, such as two hours a week, or Mondays and Wednesdays after class, or whatever suits you best. She is the one asking for a favour, so you get to decide the conditions. Saying something like "it only works for my schedule on X day, so I'm afraid you're on your own today, but I'll see you again next week" is perfectly acceptable. Trading is also completely acceptable: "I'll help you with maths if you give me some pointers for my history essay / look over my Spanish grammar / whatever she can help you with".
If you don't want to spend any time with her, then responding with "I'm sorry, I don't have time for that today. But I'm sure the teacher will help if you're stuck." will usually do the trick for me. If they persist, then raise your hand and tell the teacher that [girl's name] is really stuck and needs help with the homework. Let the teacher speak to her. People usually don't pester you again after that in my experience.