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**emakarov** First, in b), we don't have 1 in the universe; we have $\displaystyle \{1\}$, i.e., $\displaystyle \{\{\emptyset\}\}$. Recall that we are checking whether the formula $\displaystyle \forall x,y.\,x=y\leftrightarrow\forall z.\,z\in x\leftrightarrow z\in y$ is true in our universe. Therefore, $\displaystyle x$ and $\displaystyle y$ range over (i.e., take values from) our universe, which is $\displaystyle \{0,\{1\},2\}$, or $\displaystyle \{\emptyset,\{\{\emptyset\}\},\{\emptyset,\{\empty set\}\}\}$. So you should compare (as one of three pairs) $\displaystyle x=\emptyset$ and $\displaystyle y=\{\{\emptyset\}\}$.