Problem $3$: $(20\%$ of Assignment Value$)$

In a diffraction experiment, a coherent $695$ nm light source illuminates slits in a diffraction grating, and the resulting pattern is projected on a screen that is separa from the barrier by a distance of $1\mathrm{.}15$ m $.$ A student made a sketch

of the positions of the bright lines.

Calculate the density of lines, per millimeter, for the diffraction grating used in the experiment. $(1/$d $=?$ mm$)$