W = {0, {0}}。

1. Count the number of distinct elements in $W$

$W=\{0,\{0\}\}$ has two distinct elements: $0$ and $\{0\}$.

Therefore, $n(W)=2$.

1. Identify the elements of $W$ and list all subsets systematically

$W=\{0,\{0\}\}$ has two distinct elements: $0$ and $\{0\}$.

The possible subsets are
$\varnothing,\ \{0\},\ \{\{0\}\},\ \{0,\{0\}\}.$

1. Recognise the shaded region as outside both circles $A$ and $B$

The shaded region is in $\xi$ but not in $A$ or $B$.

So it is the complement of $A\cup B$.