Question about ndesu form.

So I'm doing my hw and my textbook doesn't tell me how I put a past tense negative i adjective word into a ndesu form. For example, the word muzukashii. I'm thinking it's muzukashikunakattandesu. But that just looks wrong. And when I type that into google, there are no results. So I'm thinking that's not how I do it. Can someone help me with this please?

Your answer is correct, but in romaji you won't find many results indeed. Try typing it using a Japanese IME and searching with the result: 難しくなかったんです.

Yeah, that's right. You can find the ordinary past negative form version "muzukashikunakatta desu" (without the explanatory "no") even in romaji, though.
Google