Answer:
You can write "Las sillas duras" for the first case and for the second one "Los libros azules" .
Step-by-step explanation:
"Las sillas duras"
"Las" is the article that accompanies "sillas" which is feminine and is plural because "sillas" and "las" have "s".
"Los libros azules"
"Los" is the article that accompanies "libros" which is masculine and is plural because "libros" and "los" have "s".