Learn Polish: Chcę wynająć swój pokój za trzysta złotych. - I want to rent out my room for three hundred zlotys.

Higher numbers in Polish often blend into a single word, but they change form slightly depending on the math. While sto is 100, the word for "hundreds" after 2, 3, and 4 is sta. So, 300 becomes a single combined word: trzysta (trzy + sta).

Why do we say trzysta złotych and not trzysta złote?

In Polish, numbers 5 and above (including round hundreds like 300) require the noun that follows them to be in the genitive plural form. That is why the plural word for zlotys (złote) changes to its genitive plural form, złotych. You are literally saying "three hundred of zlotys".

Does wynająć mean to rent from someone, or to rent out to someone?

It actually means both! Polish uses the exact same verb, wynająć, whether you are the tenant paying to live somewhere or the landlord renting a space out. You have to rely on context to know which is meant. Here, because the speaker is offering their own room, it clearly means "to rent out".

Could I say mój pokój (my room) instead of swój pokój?