Find the outer measure of $[-1,2]\cup \{3\}$.

Question

As we know, the outer measure $m^*$ of union of two sets $A$ & $B$ is given by $m^*(A\cup B)=m^*(A)+m^*(B)-m^*(A\cap B)$. If $A$ and $B$ are disjoint then $m^*(A\cap B)=0$. Therefore, $m^*(A\cup B)=m^*(A)+m^*(B)$. Here, $[-1,2]$ and {3} are disjoint. Then
$m^*([-1,2]\cup \{3\})=m^*([-1,2])+m^*(\{3\})$ $=3+0=3$ , because the outer measure of finite set is zero. Is this process correct?