Two slit interference question dont know how to begin

[Dars]

Homework Statement

Two slit interference question: A screen is 2.2m from two slits. The distance from point P to point R (the central maximum to the 2nd dark band) is 1.6mm.
This is what the pattern looks like (l) means light band... (d) means dark band


......P...R
(d) (l) (d) (l) (d) (l) (d) (l) (d) (l) (d)

Homework Equations

Determine the distance between the slits in terms of wavelength (lamda)

The Attempt at a Solution

I don't know how to begin, so I will say what I know... The bands are produced because the light waves interfere either constructively or destructively, depending on path length difference (difference in distance from slit to screen for any two waves) , which effects the phase difference between each waveform. If path length difference is half the wavelength this will produce destructive inteference, because the amplitudes sum and will therefore cancel, producing dark band. If path length is whole number multiple of wavelength, it will produce light band -max intensity, because the amplitudes sum and intensity is proportional to amplitude.

I also know that phase difference is path length difference divided by wavelength times 360deg.

Homework Statement

Homework Equations

please help me to begin??

The Attempt at a Solution

 

[Kurdt]

Persumably if you've studied this you will have seen something of the following.

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/slits.html#c1