Answer
$50$ decibels
Work Step by Step
$L(10^{-7})=10\log{\left(\frac{10^{-7}}{I_0}\right)}$.
Here $I_0=10^{-12}$, hence after substituting that in:
$L(10^{-7})
\\=10\log{\left(\frac{10^{-7}}{10^{-12}}\right)}
\\=10\log{\left(10^{-7+12}\right)}
\\=10\log{(10^5)}
\\=10\cdot5
\\=50$
