Learn German: Kennst du die Einkommensgrenze? - Do you know the income limit?

This is a linking element (called a Fugenlaut). It connects the two nouns das Einkommen (income) and die Grenze (limit) to make the word flow better. While there are few absolute rules for linking elements, neuter nouns ending in -en that originate from verbs (like das Einkommen) very frequently use an -s- to connect to the next word.

Since Einkommen is neuter (das) and Grenze is feminine (die), how do we determine the article for the combined word?

Compound nouns always take the gender of their final word. Because the final word is die Grenze, the entire compound is feminine, which gives us die Einkommensgrenze. The first part of the word simply describes the final part.

Why is kennen (kennst) used here instead of wissen (weißt) for "to know"?

You use kennen when you are asking if someone is familiar with a specific noun, entity, or concept (it takes a direct object, like die Einkommensgrenze). You use wissen when expressing knowledge of a fact, which is usually followed by a question word or a clause. For example, if you used wissen, the sentence would look like this: Weißt du, wo die Einkommensgrenze liegt? (Do you know where the income limit lies?).

How would I make Einkommensgrenze plural? Does the linking -s- change?