Answer
$90$ decibels
Work Step by Step
$L(10^{-3})=10\log{\left(\frac{10^{-3}}{I_0}\right)}$.
Since $I_0=10^{-12}$, then
$L(10^{-3})
\\=10\log{\left(\frac{10^{-3}}{10^{-12}}\right)}
\\=10\log{\left(10^{-3+12}\right)}
\\=10\log{10^9}
\\=10\cdot9
\\=90$
