so first we find the gap between the slits by the formula d=1/N
N is number of lines per metre so 3700 line/cm = 370000 lines/m
So d=2.7*10^-6
Now we use the formula dsin(angle)=n(wavelength)
d is the same
n is the order of the diffraction pattern
so wavelenth=dsin(angle)/n
=[(2.7*10^-6)*sin30]/3
=4.5*10^-7 m
