TeX source:
\begin{array}{rclcl} \mathbb{D} &=& \mathbb{R} \cr \cr \dfrac{1}{3}x^4+12 &=& 0 & \vert & -12 \cr \dfrac{1}{3}x^4 &=& -12 & \vert & \cdot 3 \cr x^4 &=& -36 & \vert &\pm\sqrt[4]{} \cr x_{1,2} &=& \pm\sqrt[4]{-36} \end{array}