Darling, this is about you. You need to write something that happened to YOU. There's no way people on here can do this assignment for you or even try to help when it's this simple. I'm sorry, but don't you have anything you can reflect over? A homeless person, a friend who's parents were or weren't together? Just take some time and think about it.
Answer: nine years.
Explanation: according to the given prompt, the tree doubled its height each year, therefore we can also say that no matter what year we are in (in the ten years limit), the year before, the tree was half of its size. So, if the tree reaches its maximum height in ten years, that means that the year before that (year number nine), the tree was half of its maximum height. So the answer is nine years.