TeX source:
\begin{array}{rclcl} \mathbb{D} &=& \mathbb{R} \cr\cr e^x \cdot e^{2x} -e^{18} &=& 0 &\vert& \text{1. Potenzgesetz} \cr e^{x+2x} -e^{18} &=& 0 &\vert& +e^{18} \cr e^{3x} &=& e^{18} &\vert& \ln() \cr 3x &=& 18 &\vert& :3 \cr x &=& 6 \cr\cr \mathbb{L}&=&\left\{6\right\} \end{array}