I'm not a teacher, and I do understand your text, but would have said:
"What I did was just combine difficulty and ease" (if that is possible!).
"What I done" is understandable, but incorrect.
Many Brits would commit the unforgiveable sin of placing "just" between "to" and "combine". This is called a "split infinitive" and, in my day, was pounced upon by any and every teacher of English (but, unfortunately, it's a regular mistake that we Brits make!). Dropping the "to" avoids the problem in this case.
There's an old saying: "If at first you don't succeed, CHEAT!".
Unfortunately, you can't get away with it every time.
Student or Learner