Originally Posted by **Legion**
I think that I need to somehow determine an angle from the figure and then use it in b*sin(x) = m*λ to solve for b. |

Close, but the formula is this:

$\displaystyle \frac{s d}{D} = \lambda$

where:

s = slit width

d = distance between peak of diffraction pattern and the first minimum

D = distance between slit and screen

$\displaystyle \lambda$ = wavelength of light

A quick google showed this website, which explains things in more detail

SINGLE SLIT DIFFRACTION PATTERN OF LIGHT