Is switching on the emergency lights and placing a warning triangle at the distance of 50-70 m behind the vehicle the right way to indicate that it has broken-down on a motorway?

No, the given distance is incorrect for a motorway. According to Art. 50 para. 2 of the Road Traffic Act, on a motorway or expressway, the warning triangle must be placed at a distance of 100 m behind the vehicle. The greater distance is necessary due to the much higher speeds, which gives other drivers more time to react safely.