The interference of light and diffraction

[Sox281212]

Homework Statement

https://fbcdn-sphotos-h-a.akamaihd.net/hphotos-ak-xft1/v/t1.0-9/11161364_10206082495958864_2429387894279713850_n.jpg?oh=40982e4e4e7647918991060b6f630acc&oe=55C1808A&__gda__=1439347320_8abd4b5ce66baee04bbc345aef15ec68
In terms of the wavelength of the light λ, what is the value for the path difference x = r1 – r2 if at the point P on the screen there is:

a. a dark region (and no other dark region between O and P)?

b. a bright region, with one other bright region between O and P?

Homework Equations

x = Δr = d sinƟ

The Attempt at a Solution

I don't understand this at all? could someone give me a pointer to begin with please, thanks!

 

[Sox281212]

update:
okay i think for part B, Δr= 2λ ? could someone confirm if this is correct or incorrect?? thanks!

 

[SammyS]

update:
okay i think for part B, Δr= 2λ ? could someone confirm if this is correct or incorrect?? thanks!

Yes.