See Reference types - Learning Rust
// The exact lifetime of `'a` is determined at each call site. We'll explore
// what this means in more depth later.
//
// The lifetime of `b` works the same, we just didn't give it a name.
I would write the first part as
// The exact lifetime of `a` is determined at each call site.
or
// The exact lifetime `'a` is determined at each call site.
Note, that in the first version I have removed the ' so that we refer to the variable directly, and that I have removed in the second version the "of", so that we refer to the generic lifetime directly. (a lifetime of a generic lifetime would be a bit strange?)
Perhaps we should actually write
// The exact lifetime `'a` of parameter `a`is determined at each call site.