Just include NUnit.Framework namespace in the TestClass.

The function isn't static in the default code. Most C# katas use static functions. This one, for reasons unknown, apparently doesn't. In any case, removing static from the code will work.
Although, your code still won't pass because it is wrong. Maybe you should reread the description to make sure your function is doing what the kata asks for.

Oh, the the double backslash must have been converted to a single one by the editor. Ayways, glad you found the problem.

• I don't see the using System.Linq; in your code. How exactly are you using it?
• \ needs to be escaped by another \.
• When sharing your code in a comment, you should use proper formatting, like

```csharp
public class Kata {
}
```