Yeah having an open and honest conversation seems like the best thing, but that requires Parent here also understanding it, but that's a good time for both of them to look into it and learn more.

The biggest thing is going to be something along the lines of "I know you're going to want to use this for homework, but I want to ask you to please not just use it as a way to get the answer. This very well may be a tool that helps you understand problems better, we all learn differently and maybe this can help explain confusing questions to you in a way that you can understand, but if you just ask for the answer you won't learn anything".

Being honest with them on why you don't want them to just plain use it will go a long way. Teens are (in some ways) smart, they know if you're just forbidding it that there's probably a reason but that you don't want to explain it, and so they'll rebel and use it more. Being honest and explaining your reasoning will usually sit longer with them. Sure, they'll probably say "What is X?", but maybe they'll adjust it to say "Can you show me how to solve for X?"