5 (b) (lambda*D) / a 
use this formula  
no, your not meant to use that for this question.. its quite confusing actually...
what I have done so far is as follows:
path difference between S1 and S2 = 28 cm
 v = f x (lambda)
S1   330/1000 = 33 cm
S2   330/4000 = 8.25 cm
after this I tried using  (n + 1/2)(lambda)  but I'm getting a wrong answer  
