Answer
$1.0\times 10^{-10}\frac{W}{m^2}$
Work Step by Step
We can find the required intensity as follows:
$20dB=10log\frac{I}{I_{\circ}}$
$\implies I=I_{\circ}10^{\frac{20}{10}}$
We plug in the known values to obtain:
$I=(1.0\times 10^{-12})(10^2)=1.0\times 10^{-10}\frac{W}{m^2}$
