you must try to find if each word could find letters in sequence and that the sum of the two parts is equal to s

Yes. That's why it's ranked as a 5 kyu kata. It's not completely straight forward.

A hint could be that if you don't know which part to use, try both.

If you get completely stuck, you can always forfeit your experience and look at the existing solutions. While leveling up is nice, the main goal is learning and improving, right? :-)