"What is the value of that car?"
Translation:Quelle est la valeur de cette voiture ?
There is a subtle difference in French between using "qu'est-ce que" and "quel":
"Qu'est-ce que _ ?" asks for a definition.
It makes sense in sentences like "Qu'est-ce que la vie ?" (What is life ?), "Qu'est-ce que l'amour ?" (What is love ?).
Although your sentence "Qu'est-ce que la valeur de cette voiture ?" is grammatically correct, it sounds like you're asking for the definition of the value of this car, which sounds quite weird and a bit silly.
Instead you want to ask for the value of the noun you're asking about, already knowing its definition. And this is exactly what 'quel' is used for:
"Quelle est la valeur de cette voiture ?" is the correct way to ask.
Another example would be "Quelle est la différence entre...", here "Qu'est-ce que" is not appropriate, because everyone knows what the definition of the difference between two things is.
On a side note, in colloquial spoken French, the distinction is often lost as both expressions are replaced by "c'est quoi":
for instance "C'est quoi la vie ?", "C'est quoi l'amour ?", "C'est quoi la valeur de cette voiture ?" (though it would be more common to say "Elle vaut combien, cette voiture ?").
This is however probably not accepted on Duolingo, as it is improper French.