Please prove this result
The number of ways in which n identical items can be divided into r groups so that no group contain less than m items and more than k$(m<k)$ is
=Coefficient of $x^n$ in $(x^m+x^{m+1}+...+x^k)$$^r$
In my textbook only this result is given it's prove is not given.Maybe because Binomial Theorem is our chapter next to Permutations.
But I want to know this proof because I am not able to memorize math results or formulae without knowing it's proof.Please help!!!
My attempt: I have no clue about Binomial Chapter I just know basic results and formulae.That's why I can't move even a step.Please don't close my question.