Answer:
d = 1.19 x 10⁻⁴ m = 0.119 mm
Step-by-step explanation:
This problem can be solved by using Young's double-slit experiment formula:

where,
Y = fringe spacing = 32 mm = 0.032 m
L = slit to screen distance = 6 m
λ = wavelength of light = 633 nm = 6.33 x 10⁻⁷ m
d = slit width = ?
Therefore,

d = 1.19 x 10⁻⁴ m = 0.119 mm