Find the didget that makes each number divisible by 9.

42 ? 97
What should I do in this problem?

Do you mean that you need to find the greatest common divisor of 42 and 97?

I'm pretty sure it should be "how do I know that these numbers are divisible by 9?".

And that's something I had in my elementary school book...

Pictures always helped me in elementary school ;)

Indeed, and not only that, they helped every single time you tackle a problem!