Ricardo took the following steps to simplify the expression below but was surprised to hear that his answer was incorrect. which of the following best describes ricardo's mistake?
2x+3x(x-3)-4

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 .