Croot, Lev and Pach used a new polynomial technique to give a new exponential upper bound for the size of three-term progression-free subsets in the groups $(\mathbb Z _4)^n$. The main tool in proving their striking result is a simple lemma about polynomials, which gives interesting new bounds for the size of subsets of the vector space $({\mathbb Z _p})^n$. Our main result is a generalization of this lemma. In the proof we combined Tao's slice rank bounding method with Gröbner basis technique. As an application, we improve Green's results and present new upper bounds for the size of difference sets in polynomial rings. We give a new, more concrete upper bound for the size of arithmetic progression-free subsets in $({\mathbb Z _p})^n$.