"Carro" is the most common word in Spanish. "Auto" is known in many countries but the normal word only in Argentina and Chile (and Uruguay?). "Coche" is used in Spain, Equatorial Guinea and another country (Uruguay?), in other places it means "cart" (moved by horses) or special types of carts, like baby-carts.
Because 'usar' is being used in its infinitive form following a conjugated verb. You can't have two conjugated verb sitting next to each other. So usar is being used here in its pure meaning form - 'to use'.
No, Yo no necesito - No, I do not need
usar - to use
el carro - the car.
Well, you might have used a carrot everytime you to the supermarket. So, Duo should be clear and not confuse anyone. We all need to know where to use the correct, understand let words. Or the car rental company at the airport in Spain directs you to the trolly stand for your luggage or they wonder why you speak such a mixed up Spanish with the odd South American word (phrase) thrown in. So, I am completely with you and wonder now, whether Duolingo is a good thing now. Or will we end up with a brown source of all the colour of all the regional differences thrown into one pot called Duolingo? Even the spell checkers and the installation of a lot of software programs give you the option to choose different regional ways of using EN or ES, etc.
Its because 'Carro' is more widely used in spanish countries while in the few other countries 'Carro' means a cart.
I may be wrong but I don't think this is a double negative as 'No' and 'I don't need to use the car' are two separate clauses. They emphasise and expand upon each other rather than cancelling each other out.
A double negative would be: 'I don't need no car.' Here, 'I don't need' and 'no car' would cancel each other out to mean that you do, indeed, need a car.