The correct formula for the double-slit experiment is given by

where
y is the distance of the n-th maximum from the central fringe on the screen
n is the order of the maximum
D is the distance between the slits and the screen
d is the distance between the two slits
In our problem, we are given:

, the wavelength of the light

, the distance of the screen from the slits

is the distance of the first maximum (n=1) from the central fringe
Therefore, we can re-arrange the equation to calculate the separation between the slits:
