Thread: Function Question

I'm currently on chapter 3 of Spivak's Calculus book, which is about functions. I'm confused about what this problem is asking:
Let What is..
I don't understand how you can get an answer when f(y) isn't defined, or anything concerning y for that matter. I'm pretty sure that I'm just thinking about this the wrong way, and would appreciate some help.

If and you want to find , substitute y as x.

So unless y or any other variable isn't previously defined, you simply treat it as a number?

Correct.