If $2x = x^2$ then we have that $2x - x^2 = x(2 - x) = 0$ Hence $x = 0$ or $x =2$. But $0$ is not a natural number so that the only natural number that satisfies this is $x = 2$. For the other set on the right, as
$3x = x^2$
therefore you have that $x(3 - x) = 0$. By similar reasoning only $x=3$ can satisfy this. Hence
$\{x \in \mathbb N : x + x= x ^2\} \cup \{x \in \mathbb N: 3 x = x^2\} = \{2,3\}.$
Now I address your question. You are not right because all you have shown is that $2 \in \{x \in \mathbb N : x + x= x ^2\}$ (tell me why all you have shown is this). So you need to show that in fact if you have any element $x$ in $\{x \in \mathbb N : x + x= x ^2\}$, you have that $x \in \{2\}$ which is what I showed above.