1. Given the set of real numbers $\{1,2,3, x\}$, the maximum element of the set is equal to the sum of all elements in the set, then the value of $x$ is
$\qquad$ .
If $x$ is the maximum element, then $x=x+6$, no solution; if $x$ is not the maximum element, then $3=x+6 \Rightarrow x=-3$. Therefore, $x=-3$.