Optics problem

Really need desperate help with this one:

Imagine that you have an opaque screen with three horizontal very narrow parallel slits in it. The second is a center-to-center distance a beneath the first, and the third is a distance 5a/2 beneath the first. (a)Write a complex exponential expression in terms of δ for the amplitude of the electric field at some point P at an elevation θ on a distant screen where δ=ka sinθ. Prove that I(θ) = I(0)/3 + 2I(0)/9 (cos δ + cos 3δ/2 + cos 5δ/2)
Verify that at θ=0, I(θ)=I(0)

I think i got the first part (a) right for the exponential expression. But my problem is that i don't know how to prove this Intensity equation. I tried to play around with the trigonometry identities but still dunno what i'm doing.

So any help would be greatly appreciated...