Finding 2nd Dark Fringe Angle for Diffraction Single Slit 1.5x10^6m

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A single slit has a width of 1.5 x 10^6 m and is used to form a diffraction pattern. Find the angle that locates the second dark fringe when the wavelength of the light is each of the following.

(a) 425 nm

(b) 650 nm


I answered with (a) 16.46 and (b) 25.68 but these answers are wrong
 
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You're asked for the second dark fringes, which I suppose means n = 2 in the equation
d \sin\theta = n\lambda
(you've apparently used n = 1)
 
To solve this, I first used the units to work out that a= m* a/m, i.e. t=z/λ. This would allow you to determine the time duration within an interval section by section and then add this to the previous ones to obtain the age of the respective layer. However, this would require a constant thickness per year for each interval. However, since this is most likely not the case, my next consideration was that the age must be the integral of a 1/λ(z) function, which I cannot model.
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