Translation:The car's speed is eighty kilometres an hour.
"the speed of the car" is correct and one of the accepted translations. If you only show us 3 words of a sentence we have no way of knowing what your error was.
We moderators cannot see your sentence therefore before you posted your comment here you should have made a REPORT.
Here's how to report.
Go to the bottom of the exercise page where you'll see REPORT Click on that:
Then choose what you want to REPORT: (You'll see these to choose from.)
-The audio does not sound correct.
-The dictionary hints on hover are wrong or missing.
-The Greek sentence is unnatural or has an error.
-The "Correct solution" is unnatural or has an error.
-My answer should be accepted. This is the one you should click on if your translation was not accepted.
After choosing the REPORT you want to make you should post on the comment page.