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PLUS

# I can't solve this.

Any help? Thanks!

higherOrderFunctions.swift
```/**
For this code challenge, letβs define a math operation as a function that
carries out some work on two integers and returns an integer as well. An
example is the function below, `differenceBetweenNumbers`, which takes two
integers and calculates the difference between the numbers. After calculating,
it returns the difference.
*/

func differenceBetweenNumbers(a: Int, b:Int) -> (Int) {
return a - b
}

func mathOperation((differenceBetweenNumbers: (Int, Int) -> Int, a: Int, b: Int) -> (Int,Int) -> (Int) {
return differenceBetweenNumbers(a, b)
}
```

PLUS

First of all, the return type of `mathOperation` is wrong. `mathOperation` should only return a single Int, and so does `differenceBetweenNumbers`. Therefore `mathOperation` should have a return type of `-> Int` instead of `-> (Int,Int) -> (Int)`.

Moreover, it is not a good idea to name the first parameter of your `mathOperation` function `differenceBetweenNumbers`. That is because there already is a function named `differenceBetweenNumbers`. Within the scope of your `mathOperation` function, you are "shadowing" the outside function name with the parameter name, thus it is not possible to call the "outside" function anymore.

That might not be a problem with this example, but in general you should not use the same name for variables/constants/parameters and functions. so the correct function should look something like this:

```func mathOperation(mathOp: (Int, Int) -> Int, a: Int, b: Int) -> Int {
return mathOp(a, b)
}
```

Hope this helps! Happy coding :)

The call looks good! Make sure your `mathOperation` return type doesn't have parenthesis, like in my answer. Though this is not wrong, code check here seems to be picky about it.