We will use the principle of overlap, specifically the principle of constructive interference to solve this problem. Mathematically this can be expressed as
Where,
N = Number of fringes or number of repetition of the spectrum
d = Distance between slits
Wavelength
Diffraction angle
Our values are given as
600nm
Replacing we have that the angle is,
Therefore the second order line occurs at a diffraction angle of 44.9°