This is a question that you will need to answer for yourself. My thinking about world history did not change that much when I read this book, but I was a 28 year-old teacher when this book came out. You are presumably a student and your prior thinking about world history is probably different than mine was when this book came out. What I will try to do in this answer is to help you figure out what topics to address in your own answer. I will give you guidelines for what to write about, but the details and opinions will need to be your own.
Jared Diamond wrote Guns, Germs, and Steel because he wanted to understand the “broadest patterns of human history.” He wanted to figure out why people from Eurasia came to dominate the world while people from the Americas, Australia, and Africa did not. Diamond argues that this happened because of geographic luck. It happened because people in Eurasia happened to live in a place where there were many domesticable plants and animals and where it was possible for agriculture to diffuse rapidly on a long east-west axis. Thus, people from Eurasia became dominant because they were lucky, not because they were culturally better or more intelligent or in any other way superior.
In answering this question, then, you need to think about how you would have answered Yali’s question before reading the book. If someone asked you why white people had so much more wealth and power than people from New Guinea, how would you have answered? This is the main thing you are being asked to think about in this question. Would you have said white people were genetically better? Would you have said that their culture was more interested in making progress than other cultures? Would you have had any answer at all? How is your answer different now that you have read the book?
Another aspect of the question could have to do with the idea of proximate and ultimate causes. If you were asked Yali’s question, would you have said that white people were richer and more powerful because they had more technology? Would you have said that they conquered the Native Americans because of their technology and because diseases wiped out so many of the Indians? If so, you were only thinking about proximate causes. Diamond wants you to think about ultimate causes. He wants you to think about why the Europeans came to have the “guns, germs, and steel.” This might be another way in which your thinking could have changed.
What I think your instructor wants to see is that you know what Diamond says in this book and that you are thinking about it. If I assigned this question, I would want you to talk about Diamond’s theory about why Europeans came to dominate the world. I would want you to tell whether/how the book has changed how you think about this topic. I would be impressed if you went beyond that to talk about the idea of proximate and ultimate causes. Before you read the book, would you have thought about ultimate causes or only about proximate causes?
If you discuss these topics, your answer should be good.