It is suggested to establish confidence in doing math problems first, starting with examples in textbooks and after-class exercises, and starting with simple topics. Don't do too many questions at once or one day, just one or two. Confidence came when she found that these topics were all talked about by the teacher and were easy to do.
Once again, begin to strengthen, and the topic should still be based on textbook exercises. At this time, you should increase your exercise, but at this time, you should pay attention to your niece's mentality and avoid blindly adding a lot. If your niece is a little upset, jump off the math problem in time and move on to something she is very interested in.
Finally, in her opinion or yours, she has mastered the general exercises in the textbook. At this time, she can expand after class. Mainly step by step, pay attention to your niece's mentality.
Ok, that's all, I hope to adopt it.