My try:
Suppose $x \in A \cup (B - (A \cap B))$.
If $x \in A$, then $x \in A \cup B$.
If $x \in B - (A \cap B)$, then $x \in B$ and $x \notin A \cap B$.
Specificaly, $x \in B$. Hence $x \in A \cup B$.
Then, by subset definition, we have $A \cup (B - (A \cap B)) \subseteq A \cup B$.
Yes your proof is correct. You have showed the steps required for the solution.