Hello
$\displaystyle
\begin{cases}
85=C\exp(3b) & (1)\\
13=C\exp(5b) & (2)
\end{cases}$
As $\displaystyle \exp(5b)\neq 0$, one can divide (1) by (2) which will give an equation that can be solved for $\displaystyle b$. Then use either (1) or (2) to find the value of $\displaystyle C$.