The liver acts to keep blood glucose at a relatively constant level. it takes up glucose from the blood when levels are too high, and adds glucose to the blood when levels are too low. are these actions of the liver an example of homeostasis?

Yes, these actions are an example of homeostasis. To fully understand this question you need to know the basic concept of homeostasis. Homeostasis is the body's regulation. What do I mean by that? It's simple, when you have a fever your body's temperature can skyrocket to 100 or more, when you start to sweat or get the chills that's your body's way to cool you down and fight the infection. Another example; when you do any physical activity, you sweat from it depending on the intensity. If you didn't sweat you would probably overheat and that would lead to other complications. Basically when you do exercise you sweat because your body is warm and that's the homeostasis mechanism that helps cool down your body bring it to it's normal temperature. So in end answering your question in more detail by the liver regulating the body by taking glucose in when levels are too high or adding glucose in when levels are too low is an example of homeostasis because the body is regulating itself to come back to it's normal healthy function state.

Hope this helped :)