Answer
a). $100dB$
b). $60dB$
c). $-30dB$
Work Step by Step
a). Intensity = $10log\frac{I}{I_{0}} =10log\frac{10^{-2}}{10^{-12}}=100dB$
b). Intensity = $10log\frac{I}{I_{0}}=10log\frac{10^{-6}}{10^{-12}}=60dB$
c). Intensity = $10log\frac{I}{I_{0}}= 10log\frac{10^{-15}}{10^{-12}}=-30dB$