。
Obeying the law is everyone's duty.
Literal
Law [dat-に] obey [fact-こと] [topic-は] everyone [possessive-の] duty-[assertion-だ]
Here こと nominalization is used instead of の. The two are often interchangeable; こと feels slightly more abstract or formal, の more concrete and colloquial. The pair is worth a side-by-side glance to feel the nuance.