Translation:These letters and books are mine.
Saya punya surat-surat dan buku-buku ini = I have these letters and books. Surat-surat dan buku-buku ini punya saya = These letters and books have me. This is how ownership can be expressed in Indonesian. Quite literally the thing has (owns) you. That is how it is expressed. Weird I know, but do not expect everything to be equivalent in another culture. If you accept this as how it is, then there is no problem. Just gotta get your head to learn the way it is :-)
I gave this as my solution: "These letters and books belong to me" and Duo considered it to be wrong. The correct solution, according to Duo, was: "These letters and books belongs to me."
Duo's solution has bad grammar, however. Because the nouns are plural (with an "s"), the verb should also be plural (without an "s").