[tex]\sqrt{x+12}=x\\\Leftrightarrow \left\{\begin{matrix} & x\geq 0 & \\ & x^2-x-12=0 & \end{matrix}\right.\\\Leftrightarrow \left\{\begin{matrix} x\geq 0 \\ \left[\begin{array}{l} x=4\\x=-3 \end{array}\right. \end{matrix}\right.\\\Leftrightarrow x=4[/tex]