Answer:
If the slits are separated by d then s / d where s is the difference in the wave path between opposite sides of the slit
(a diagram would be useful here)
This can be expressed by:
sin θ = (λ / 2) / d where θ is the angle of diffraction
If d is the width of the slit then
d = λ / (2 sin θ) = 6.36E-7 / (.845) = 7.52E-7 m = 7.52E-5 cm