Answer: Correct
Step-by-step explanation: Mercedes decidió comprar un auto nuevo
This sentence can be translated to:
Mercedes decided to buy a new car.
In the sentence, first comes Mercedes, that is the substantive, then comes the action, that is "decidió", and then comes in what the action is applied, in this case, she decided to buy a new car.
So the order is correct, then the sentence is grammatically correct.