a sin(theta) = m lambda destructive interferencewhere "a" is the width of the slit, and "m" an integer 1, 2, 3 ...
theta = lambda / a
theta = 1.22 lambda / D
d sin(theta) = m lambda m = 1, 2, 3, ...where "d" is the spacing of the slits
This lecture discusses material in Chapter 38 of Serway.