What is the missing variable in the double slit interference pattern?

[CricK0es]

Homework Statement

Laser light of wavelength 517 nm illuminates two identical slits, producing an interference pattern on a screen 87.5 cm from the slits. The bright bands are 1.15 cm apart, and the third bright bands on either side of the central maximum are missing in the pattern.

a.) Find the slit width, a
b.) Find the distance between their centres, d

Homework Equations

dsinθ = mλ

The Attempt at a Solution

I'm fairly sure I can do b, it's just a bit of trig. But I'm wondering how one would find a. I understand the slit width has to be comparable to the wavelength for diffraction to occur, but I'm wondering if this question demands something a little more methodical.

Many thanks

-- Yeah I got 3.93x10^(-5)m for d and that is correct [/B]

 

[mfb]

and the third bright bands on either side of the central maximum are missing in the pattern.

You'll have to figure out what that means, it will lead to the slit width.