\frac{dx_{2}}{dt} = \left(1 \cdot k_{8} \cdot k_{3} \cdot \exp\left(\left(-k_{4}\right) \cdot t\right) + 1 \cdot k_{8} \cdot k_{2} \cdot x_{2} + -1 \cdot k_{8} \cdot x_{1} \cdot x_{2}\right) / k_{8}