The empty set is φ, without parentheses. And {φ} this set contains an element φ, so this set is not an empty set. In the case of ab uncertainty, X has multiple solutions, so it is not a single element set. According to the solution of the equation, x = 5 and y = 1, so its solution set is {(5, 1)}. N stands for natural number, and when m = 0 12345678 holds, so X has nine solutions, that is, M has nine elements. I wish you progress in your study.