The formula that relates the fringe characteristics to the experimental setup is
$\displaystyle \lambda = \frac{Ws}{D},$
where:
$\displaystyle \lambda$ is the wavelength of your light source (in m);
W is the distance between successive fringe peaks, or troughs (in m);
s is the slit separation distance (in m); and
D is the perpendicular distance between the slit panel and the screen.
Using this formula, you should be able to solve your problem. Have a go and let us know how it goes.
Most exam boards supply the above formula on the formula sheet. If not, you should learn it!
