Answer
$x \approx 0.59$
Work Step by Step
RECALL:
$\log{10^x}=x$
Take the common logarithm of both sides to obtain:
$\log{10^x}=\log{3.91}$
Use the rule above to obtain:
$x=\log{3.91}$
Use a scientific calculator to obtain:
$x=0.5921767574
\\x \approx 0.59$