It is in the description and it can be seen in the example usage code. I guess I could write is extra clearly, but it is mentioned.

The kata was made for C# first of all, which is strongly typed, so there never was a question about what you need to return back then. Which programming language were you using? Maybe I can make it more clear from the inital state of the code too.

Yes it is.

Output string must be two numbers separated by a single space, and highest number is first.