Answer
$I=1\text{ watt/m}^2$
Work Step by Step
$L(I)=120$ dB
Putting in the equationȘ
$L(I)=10\log \frac{I}{10^{-12}}$
$120=10\log (I\times 10^{12})$
$\frac{120}{10}=\log(10^{12}I)$
$\log10^{12}I=12 $
$10^{12}I=10^{12}$
$I=\frac{10^{12}}{10^{12}}=1$ $\frac{watt}{m^2}$