Answer
given,
width of slit, d = 0.08 mm
d = 8 x 10⁻⁵ m
light of two wavelength
I₁= 446 nm
I₂ = 662 nm
a) angles at which the third dark fringe

m = 3 , I₁= 446 nm

C = 0.958°
m = 3 , I₁= 662 nm

C = 1.423°
b) angles at which the third dark fringe

m = 1 , I₁= 446 nm

C = 0.319°
m = 1 , I₁= 662 nm

C = 0.474°