"Matěj is our friends' son."
Translation:Matěj je syn našich přátel.
Check the genitive plural in the declension table https://en.wiktionary.org/wiki/p%C5%99%C3%ADtel#Declension There are more nnouns that behave this way, there is an example list http://prirucka.ujc.cas.cz/?id=227
Depends on your device, but ask about it elsewhere, not in a sentence discussion. A search engine like Google is your friend http://duolingo.wikia.com/wiki/Guide_to_keyboard_layouts_and_input_methods
for this exact same question in another part of this lesson, "Matěj je syn našich přátel." is rejected and this is given as the only correct answer: ""Matěj is our sons' friend." Translation:Matěj je přítel našich synů."
edit: my question was the result of failing to read the subtle differences between two sentences. i am tuning my eyes to read more carefully now!
So, I'm a little blurry on this word order. I put "Matej je pratel nasich syn" (sorry, no Czech keyboard) which reflects the JUST previous exercise of "Zofie je pritelkyne nasich dcer" and yet it was marked WRONG. I don't mind being wrong as long as I understand why and can learn from my (many) mistakes.