A screen is placed 50cm away from a slit lit by a monochromatic light of wavelength 690nm. The distance seperating the first dark fringe and the third dark fringe in the diffraction pattern is 3mm. Calculate the width of the slit.

Answer 1

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/sinslit.html
Well, you have y1-y2 as 3mm
and m1+1=m2

so in the two equations for the dark minimus, subtract one from the other to end with one equation using your indicie and the 3mm.