Physics double slit waves which formula for ∆s?

Hello Good Question Community,

We're having a physics exam soon, and there are a few things about waves that I just don't understand. I've looked at several notes and videos, but unfortunately, they haven't helped me.

I have calculated various tasks for the double slit, but in these ∆s were sometimes replaced differently and I do not understand the difference:

1: △s = n * lambda/2

2: △s= (n-1/2) * lambda

3: △s= (2n+1) * lambda

When do I use which one and what are the differences?

Thank you very much for all your efforts