Answer
$I=398.11$ cm
Work Step by Step
$M=6.6$cm
$S=10^{-4}$cm
Putting in the equation:
$M=\log\frac{I}{S}$
and solving for $I$:
$6.6=\log \frac{I}{10^{-4}}$
$\log(10^4I)=6.6$
$10^4I=10^{6.6}$
$I=\frac{10^{6.6}}{10^4}=10^{2.6}\approx 398.11$ cm