Answer
$L=6.64\mu m$
Work Step by Step
We know that:
$N_2-N_1=(\frac{L}{\lambda})(n_2-n_1)$
This can b e rearranged as:
$L=\frac{(N_2-N_1)(\lambda)}{n_2-n_1}$
We plug in the known values to obtain:
$L=\frac{7\times 550\times 10^{-9}}{1.58-1.0}=6.64\times 10^{-6}=6.64\mu m$
