Answer
$\not \in$
Work Step by Step
The symbol $\in$ means that the number/expression is an element of the set.
The symbol $\not \in$ means that the number/expression is NOT an element of the set.
Note that $\left\{2\right\}$ is a set.
Since $\left\{2\right\}$ is not an element of the set on the right, then the missing symbol in the blank is $\not \in$.
