Analyzing the elements in X it follows that they are even integer numbers.
Any of the terms of X may be found using the formula x = 2z + 2, z in Z.
The pattern 2z + 2 is specific to the even numbers.
You may verify this statement plugging integer values instead of z in the relation x = 2z+2.
z = -1 => x = -2 + 2 = 0
z = 0 => x = 2
z = 1 => x = 2+2 = 4
Notice that the results are integer even numbers.
Analyzing the elements in Y it follows that they are odd integer numbers.
The union of X and Y is a set that collects elements from X or elements from Y. This collection of odd and even integer numbers represents the integer set Z.