Aloha :)
$$\left.f(t)=8500\quad\right|\;\text{einsetzen}$$$$\left.2\,e^{1,6094\,t}=8500\quad\right|\;:2$$$$\left.e^{1,6094\,t}=4250\quad\right|\;\ln(\cdots)$$$$\left.1,6094\,t=\ln(4250)\quad\right|\;:1,6094$$$$\left.t=\frac{\ln(4250)}{1,6094}=5,1912\quad\right.$$