Chapter 7 - Exponential and Logarithmic Functions - 7-6 Natural Logarithms - Practice and Problem-Solving Exercises - Page 483: 74

$x=\dfrac{10,000}{3} \approx 3,333.3333$

Recall: $$\log_a{b}=y \longleftrightarrow a^y=b$$ Use the definition above to obtain: \begin{align*} \log{3x}&=4\\\\ 10^4&=3x\\\\ 10,000&=3x\\\\ \frac{10,000}{3}&=\frac{3x}{3}\\\\ \frac{10,000}{3}&=x \end{align*}