How can I find the width of the central diffraction maximum.

[MattDutra123]

If I am given the width of the slit (b), wavelength of the light (λ), and the distance of the slit from the screen (D), how can I find the width of the central maximum (d)? My book says d/2=Dλ/b, but with no explanation and I don't understand why. Where does this formula come from?
Thank you for your help.

 

[Doc Al]

Start here: Fraunhofer Single Slit