x_{1} = k_{4} / k_{2} - \left(k_{1} / k_{2} + k_{4} / k_{2}\right) / \left(1 + k_{1} / \left(k_{2} \cdot k_{4} / k_{2}\right) \cdot \exp\left(\left(k_{1} + k_{2} \cdot k_{4} / k_{2}\right) \cdot t\right)\right)