BEFORE YOU BUY!
TRY TO BORROW FIRST!!!
Go to your school library or local library and try to find a physical or digital copy and just borrow that. Textbooks are expensive for most people and are usually a waste of money unless you can afford to just straight up buy one without any issues.
Cambridge Mathematics Extension 2
New Senior Mathematics Extension 2
Terry Lee New Advanced Mathematics Year 12 Extension 2
https://www.advancedmathematics.com.au/products.htm
Maths in Focus Extension 2 (Personally don't like the questions in this one but use it if you have to)
https://www.fivesenseseducation.com.au/maths-in-focus-extension-2-year-12-student-book-9780170413435