"Szia Péter, a rendőrtől jössz?"

Translation: Hello Péter, are you coming from the police officer?

[Eni892714]

The english sentence is not the same here and in the excercise, which says "Hi Péter are coming from the police officer". So a "you" is missing.

[_paranoia_]

Yes, it's not correct without the "you" in this example. I guess the difference is that Péter is not the subject of a verb in the first clause, as you can tell by the "are" in the second clause. E.g., "Péter, are you eating lunch and coming from the police officer" would be perhaps ok.

[Giuseppe297424]

Would "...from the policeman's/police officer's" be acceptable? meaning from his/her house?

[JohnWeber786406]

There should not be a problem

[JohnWeber786406]

I didn't hear the article "a" before rendőrtől.

[OscarFoong]

What's the difference between "did you come" versus "are you coming"?

[MrrzyV]

Tense. Did you come is past, are you coming is present.