Let’s look at the sentence to which you are referring:
Juanita used her affinity for numbers to help her design a computer program that relied on college-level math formulas, even though she was only in middle school.
As you may know, context clues offer hints (intentional or not) by the author that assist readers in determining the meaning of certain words. Context clues don’t necessarily have to be in the same sentence, but in this case they are. The context clues that exist here tell us that she was able to design something that was beyond her level of education. And, she was able to do this because her “affinity” or deep interest/liking of numbers.