Answer:
y = λ L/ a
Explanation:
The diffraction pattern of a single slit is side for the case of destructive interference by
a sin θ = m λ
Where a is the width of the slit, lm the wavelength and m an integer
In general the screen is much larger than the width of the spectrum, so the sine can approach the tangent
sin θ = tan θ = y / L
Substituting remains
a y / L = m λ
y = m λ L /a
For the first dark strip m = 1
y = λ L/ a