How to approach diffraction and wave problems

[G4CKT]

Hi, I started a new assignment, the last one for my course and its hard as !@#@.

I don't know how to start these questions at all if anyone can give some guidance or formulas which can be used that would be great!

http://img11.imageshack.us/img11/9773/53830806.png

 

[G4CKT]

Can anyone help? My friends are clueless on what to do as well xD