Angular width of a fringe is given by

  $\theta =\frac{\lambda }{d}$  

$\Rightarrow d=\frac{\lambda }{\theta }$              (1)

Given $\lambda$ = 628 nm = $628\times {10}^{-9} m$  and $\mathrm{\theta }=0.1°= \frac{0.1\times \mathrm{\pi } }{180}$rad.

Using these values in Eq.(1), we find that $d=3.6\times {10}^{-4}$m