Double slit diffraction has a convenient formula for this of
[tex]s=wD/d[/tex]
Where s is the distance between the fringes, w is the wavelength (should be lambda but this website doesnt seem to like it in the formula construction box), D is the distance slit to screen, and d is the distance between the centres of the two slits.
Substituting this all in:
[tex]s= \frac{(490E-9)*4}{2E-4} = 0.0098m[/tex]
