Learn Portuguese (Portugal): Um carro com uma quilometragem alta não é fiável, pois não? - A car with high mileage is not reliable, is it?

When you add a tag question like "is it?" or "right?" to the end of a negative statement, you use pois não. If the main sentence had been positive (for example, if you were saying the car IS reliable), you would use não é? instead.

Why is the adjective "alta" feminine here?

It describes quilometragem (mileage). In Portuguese, almost all nouns ending in -agem are feminine, so it requires the feminine article uma and the feminine adjective alta. Other common B1 examples are a viagem (the trip) and a mensagem (the message).

The lesson covers relative clauses like "com que". Why don't we use "com que" here?

You only use com que (with which) when introducing a new clause that contains its own verb, like in o carro com que viajo (the car that I travel with). In this sentence, com is just a simple preposition followed by a noun phrase (uma quilometragem alta), so we don't need que.

Where does the word "fiável" come from?

Fiável translates to "reliable" or "dependable". It comes from the reflexive verb fiar-se, which means "to trust". It can be used for both things (like a car) and people (meaning someone is trustworthy).