We know that $(P(X), \Delta) $ forms a group, where $P(X) $ is the power set on the non empty set $X$ and $A\Delta B=(A-B) \cup (B-A)$ for all $A, B\in P(X) $. Clearly $\emptyset$ is the identity element. How can i compute $E$ from the relation $Y\Delta E=Y$ for all $Y\in P(X) $.