I'd say it's appropriate based on the Wiki:

7 kyu kata represent a beginner programming level. At this level the kata will generally challenge users on their core language and API reference knowledge. Tasks such as:

Iterating arrays and returning a subset of values

Basic data type manipulations

Basic functional or object-oriented concepts

Basic Regular Expressions

This one meets the second and potentially the first bullet points. I agree that the language of the setup is less than clear. For example using "s1" as an example input but "a1" as the predefined function input could be needlessly confusing.

edit: on a second look, I now realize that the problem with the description is "call this x and call this y and here's a formula relating the two". Yeah, that's obnoxious. I've spent enough time in math books that I didn't really notice, but for a beginner that's a very unintuitive way to describe what should be an intuitive task.