Rewrite all the tasks in a new exercise book

Is an exercise book blank?

What do you mean?
Новая does mean 'blank'.
You add it, but correct the original. Пишут В тетради, а не НА тетради. 'НА' означает на обложке тетради.

I’m still not sure what an exercise book is by your/Russian standards. I’m thinking you mean a notebook, which is a blank book of pages that you write in. 

https://www.lingvolive.com/en-us/translate/ru-en/тетрадь
https://ru.wikipedia.org/wiki/Тетрадь - usually it has as little as 10-12 pages
https://en.wikipedia.org/wiki/Exercise_book

We just call those notebooks. When it’s time to go back to school, your parents buy them in stacks.