@adamerickson ,
This functionality does not exist in New Quizzes. It was not supported in legacy quizzes either.
There are some things that were worked with formula questions in legacy quizzes that do not work with New Quizzes. For example, you could use variable names that started with a letter and included a number. I regularly had variable names of [x1], [x2], [x3], but those are not picked up by New Quizzes.
Last year, I wrote a blog: Using Calculated Values in the Text of Formula Questions. I have not explored New Quizzes the way I have legacy quizzes, so I do not know if a similar approach could be used.
One thing that is nice about New Quizzes and formula questions is that you can include a variable within a LaTeX expression.
If your question stem is:
Evaluate \(`x`+\sqrt{`y`}\)
It gets displayed like this:

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