You have not given us any of the steps that Ricardo took to simplify the
expression, and you also haven't given us the list of choices that includes
the description of his mistake, so you're batting O for two so far.
Other than those minor details, the question is intriguing, and it certainly
draws me in.
If Ricardo made a mistake in simplifying that expression, I'm going to say that
it was most likely in the process of removing the parentheses in the middle.
Now you understand that this is all guess-work, because of all the stuff that you
left out when you copied the question, but I think he probably forgot that the 3x
operates on everything inside the parentheses.
He probably wrote that 3x (x-3) is
either 3x² - 3
or x - 9x .
In reality, when properly simplified,
3x (x - 3) = 3x² - 9x .
