"Elas anunciam seu livro."

Translation: They announce your book.

[ppmcltx]

"their book" should also be OK I suppose? though not accepted

[Paulenrique]

that's ok too!

[jah1984]

But isn't it more common in Brazil to say "their book" as "o livro delas"?

[Paulenrique]

Yes, but seu/sua = your, his, her, its, their, so his answer is still right.

[ManueldeBrito]

Why not, " Elas anunciam seus livros. " ?

[Paulenrique]

That is "your books".