The number of subsets of a finite set $A$ is equal to 2 raised to the power of $n\left(A\right)$, where $n\left(A\right)$ is the number of elements in set $A$: Number of Subsets of Set $A=2^{n\left(A\right)}$.

The cardinality of $A$ union $B:n(A\cup B)=n(A)+n(B)-n(A\cap B)$