Let $a$ and $b$ be two sets. The pair of $a$ and $b$ is
\[\{a, b\} = \{x : x = a \lor x = b\}.\]This is a set by Axiom of Pairing.
For any sets $a$ and $b$,
\[\{a, b\} = \{b, a\}.\]
Let $a$ and $b$ be two sets. The pair of $a$ and $b$ is
\[\{a, b\} = \{x : x = a \lor x = b\}.\]This is a set by Axiom of Pairing.
For any sets $a$ and $b$,
\[\{a, b\} = \{b, a\}.\]