TeX source:
\begin{array}{rclll} -15 &=& e^{7 \cdot x-15} \cr -15 &=& \dfrac{e^{7x}}{e^{15}} &\vert & \cdot e^{15}\cr -15\cdot e^{15} &=& e^{7x} &\vert & \ln() \cr \ln(-15\cdot e^{15}) &=& 7x\end{array}