"It is between seventeen and nineteen."
Translation:C'est entre dix-sept et dix-neuf.
You have to figure out what the English sentence means, especially what "it" is supposed to represent.
As a personal pronoun, "it" can be an animal or thing, which can translate to the personal pronouns "il" or "elle".
"It" can also represent a whole situation, and then there are several possibilities.
- it is + an adjective = il/elle est + adjective, like "it is red" = "il/elle est rouge."
- it is + determiner + noun = c'est + determiner + noun, like "it is a mess here" = "c'est le bazar, ici"
As an impersonal subject, "it" can be used in an idiomatic phrase and not mean anything: "it is raining", (il pleut) "it is Monday today" (c'est lundi aujourd'hui) "it is seven am". (il est sept heures). You have to learn them as you go.
The sentence here is rather vague (what is the measure? 17-19 what?), so the vague "c'est entre dix-sept et dix-neuf" is a correct translation, as well as "il/elle est entre dix-sept et dix-neuf".