Question 13: Los libros suyos son buenos. (You should just say tuyos though)
Question 14: Las clases nuestras son aburridas. (I'd say nuestras clases son aburridas, it's way simpler)
Question 15: La computadora tuya no es nueva.
(P.S. I'm from a latin country but every latin country has their own way of saying things, so the way I say it may be said in a different way somewhere else.)
