Answer
$x\approx-1.0979$
Work Step by Step
Using properties of logarithms and since $\log_b x=y$ implies $b^y=x$, then,
\begin{array}{l}
\ln(3x+7)=1.31
\\\\
\log_e(3x+7)=1.31
\\\\
3x+7=e^{1.31}
\\\\
3x=e^{1.31}-7
\\\\
x=\dfrac{e^{1.31}-7}{3}
\\\\
x\approx-1.0979
.\end{array}
