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You are correct in your analysis (though I would not call `new_b` a "parameter" but a captured variable).

What you want is for the closure (anonymous function) to take responsibility for `new_b` and not just borrow it temporarily. In other words, you want move semantics and not a borrow.

As mentioned in [Rust by Example](https://doc.rust-lang.org/rust-by-example/fn/closures/capture.html), closures borrow their captured variables by default. If you would instead like to move them, you simply need to put `move` before the parameter list of the closure. (See the bottom of the linked section.) That is:

```rust
impl MyThing {
    fn update_Fn(&mut self, delta_b: f32) {
        let new_b = self.b + delta_b;
        self.b = new_b;
        self.Internal_Fn = Box::new(move |x: f32| ((1.0 + new_b).powf(x) - 1.0) / new_b);
    }
}
```