Which of the following sets is equal to {1, 2, 3, ... }? x e R, x > 1 x x e N, x ≥ 1

Well, you know the first set can't be the answer, since it makes it such that x is more than 1:

x

and our set clearly includes 1. And since each of the numbers in the set are whole numbers, set two can not be right, because it does not specify that numbers must be whole. it only specifies that numbers must be real and more than 1. Thus the third set is the answer.