The expression shown in the image is:

$2 \subseteq \{1, 2, 3\}$

This expression is not valid because 2 is an individual element, while the symbol "⊆" (subset) is used to denote a relationship between sets. For the subset notation to make sense, both the left-hand and right-hand sides should be sets.

To make it correct, it should be something like: $\{2\} \subseteq \{1, 2, 3\}$

This is true, as the set $\{2\}$ is a subset of $\{1, 2, 3\}$, meaning that all elements of $\{2\}$ are contained within $\{1, 2, 3\}$.

Would you like further details or clarification?

Here are 5 related questions:

1. What is the difference between an element and a subset in set theory?
2. Can an empty set be a subset of any set?
3. How do we determine if one set is a subset of another?
4. What is the meaning of a proper subset, and how is it denoted?
5. Can a set be a subset of itself?

Tip: Always ensure both sides of the subset symbol are sets for the notation to be valid.