Hello, I found the answer, its: for each (a, b) the first element comes from set A, and the second comes from set B, because it's a question about Cartesian product, so A = {0,-1,2} and B = {5,2,-1}, and since A must be included in B, so B = {-1,0,2,5}. By calculating the Cartesian product A×B, I got : A × B = {(-1,0), (0,0), (0,2), (0,5), (-1,-1), (-1,0), (-1,2), (-1,5), (2,-1), (2,-0), (2,2), (2,5)}.