How do I stop getting this error?:

Error: Command failed: /tmp/csharp11641-19-oqtvya/fixture.cs(1,2): error CS0246: The type or namespace name TestFixture' could not be found. Are you missing NUnit.Core' using directive?
/tmp/csharp11641-19-oqtvya/fixture.cs(4,4): error CS0246: The type or namespace name Test' could not be found. Are you missing NUnit.Core' using directive?
/tmp/csharp11641-19-oqtvya/fixture.cs(16,57): error CS0103: The name `String' does not exist in the current context

Thanks for the help though, your remark about not seeing the using statement made me try it out in the solution box and that ended up fixing everything :)

1. I did use "\", if you couldnt see it then maybe i pasted the wrong thing.
2. I didn't know about the formatting thing
3. I found out the problem was that my using System.Linq statement was in the test cases box and it won't work that way. I had to put it in the solution box and it worked.

using System.Linq;

public class Kata
{  
  
  public static char[] noise = { '%', '$', '&', '/', '#', '·', '@', '|', 'º', '\\', 'ª' };
  
  public static string removeNoise(string equation)
  {
    char[] sentence= equation.ToCharArray();
    string newString = "";

    for (int i = 0; i < sentence.Length; i++)
      {
        if (!noise.Contains<char>(sentence[i]))
        {
          newString += sentence[i];
        }
      }
    
    return newString;    
  } // end RemoveNoise method
  
} // end class