Answer
$I = 10^{-5}~W/m^2$
Work Step by Step
We can find the initial sound intensity:
$\beta = (10~dB)~log(\frac{I}{I_0})$
$70~dB = (10~dB)~log(\frac{I}{I_0})$
$7 = log(\frac{I}{I_0})$
$10^7 = \frac{I}{I_0}$
$I = I_0 \cdot~10^7$
$I = (10^{-12}~W/m^2)~(10^7)$
$I = 10^{-5}~W/m^2$
