Learn Finnish: Opiskelija palautti kirjallisen tehtävän. - The student returned the written assignment.

The base word is kirja (book), and adding the highly common suffix -llinen turns it into the adjective kirjallinen, meaning "written" or "literary" (literally "book-related"). Because it describes the object of the sentence, it takes the accusative -n ending to become kirjallisen.

Why do both kirjallisen and tehtävän end in -n here?

They make up the complete object of the action. Because the student finished returning the whole assignment, Finnish uses the accusative case (which looks exactly like the genitive -n for singular words). Adjectives must always match the case of the noun they describe, so both get the -n ending.

I know palata means "to return." How is the verb palauttaa different?

Palata is an intransitive verb used when someone returns somewhere (like coming home). The verb used here, palauttaa, is a related transitive verb that means "to return an object" or "to give something back." Palautti is its past tense form.

The word tehtävä looks like it comes from the verb tehdä (to do). Is that right?