Perhaps what you find confusing is the use of set-builder notation to define : Included in between { ... } are the condition(s) that any "candidate" element must satisfy in order to be included in the set, and a set defined by set-builder notation contains all, and only, those elements satisfying all the conditions given.

In each of , set-builder notation is used to provide the conditions for inclusion in each set, respectively. Note: unless otherwise stipulated, you can take conditions separated by a comma to be a conjunction of conditions; that is: $$X = \{x : \text{(condition 1), (condition 2), ...., (condition n)}\}$$ means is the set of all x such that x satisfies (condition 1) AND x satisfies (condition 2) AND ... AND x satisfies (condition n).

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The only solution to are or , neither of which is odd. Hence there are elements in ; that is, .

$$Q= \{x: x^2 = 9, x \text{ is even}\}$$

The only solutions to are or , neither of which is even. Hence, there are no elements in ; that is, .

is the only solution to , but is not a solution to , (and neither nor is a solution to ). Hence, there are no elements in ; that is, .

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NOTE: As an aside, regarding notation - sometimes instead of a colon :preceding the defining characteristics of a given element, you'll see | in place of the colon. E.g.,

Hint: In this question it is given that we have to define null sets with example. So for this first we have to know about null sets, or what is a null set and why we call it a null set. Complete step-by-step answer:Definition: If a set does not contain any element or member then the set is called a null set. Null set is also called a void set or empty set. The symbol used to represent an empty set is – $\\phi$ ,{}.Examples:1) Let A = {x :1 < x < 2, x is an integer } be a null set because there is no integer between numbers 1 and 2. Therefore, A = {} or $\\phi$2) Let W = {d: d > 8, d is the number of days in a week} will also be a void set because there are only 7 days in a week.Note: Always remember that a null set implies no elements in a set but here it does not mean that the element is zero.i.e, {0}, because if you write {0} then it is not a null set , since this set has an element which is zero.