Answer
$L=5.15\mu m$
Work Step by Step
We know that
$N_2-N_1=2(\frac{2L}{\lambda})(n_2-n_1)$
This can be rearranged as:
$L=\frac{\lambda(N_2-N_1)}{2(n_2-n_1)}$
We plug in the known values to obtain:
$L=\frac{589(7.0)}{2(1.40-1.00)}=5.15\mu m$