Calculating Single Slit Diffraction Width with 630 nm Light

[muffintop]

Homework Statement

A single slit is illuminated with 630 nm light, and the resulting diffraction pattern is viewed on a screen 2.8 m away.
If the linear distance between the first and second dark fringes of the pattern is 12 cm, what is the width of the slit?

Homework Equations

y=Ltan\theta
W = \lambda m / sin \theta

The Attempt at a Solution

theta = arctan (.12/2.8) = 2.45
W = 630 x 10^-9 x 2 / sin 2.45

 

[Doc Al]

Looks good.

 

[muffintop]

I got 29.4 micrometers. It's wrong though.

 

[Doc Al]

W = 630 x 10^-9 x 2 / sin 2.45

Oops... I missed that factor of 2 before. Why is it there?

Hint: Write expressions for the first dark fringe position and the second dark fringe position, then subtract.

 

[muffintop]

I got it thank you so much!
I always get my "m" values wrong.