Theorem3. Every box of size B ≥ p+1 2 in the plane contains a solution of (1.1). Proof. Let I be the projection of the box on the x − axis, and J be the projection on the yaxis, let S = a · I = {ax : x ∈ I} and T = −b · J = {−by : y ∈ J}, then |S| ≥ p+1 2 and T ≥ p+1 2 , hence by Theorem 2 for every c ∈ Zp there exists ax ∈ S and −by ∈ T such that ax−by = c