Why use notre and not nos

Bonjour Andrew !

Unfortunately here I cannot see the example you're referring to, but here's the rule for using "notre" vs "nos" :
- if the noun following is plural, "our dogS", then you use "nos" -> "nos chiens"
- if the noun following is singular, "our cat", then you use "notre" -> "notre chat"

Here's a link to the related lesson:
https://french.kwiziq.com/my-languages/french/view/3113

I hope that's helpful!
À bientôt !