Constructive & destructive interference of light waves

[necromanzer52]

Homework Statement

A glass-plate of 350 nm thickness is free standing in space. At the angle of 75 degrees, relative to the glass-plate plane, a white light point-source is placed. The diffraction (probably a typo, and should really be refraction) index of the glass is 1.33. On the same side as the light source, a screen is placed, allowing the observation of the reflections from the glass-plate.
Derive, explicitly, the conditions for the observation of constructive and destructive interference on the screen.

Homework Equations

I really have no idea, other than; that Snell's law should probably come into it at some stage.

The Attempt at a Solution

I drew a diagram, and I used Snell's law to work out the angle of refraction. I sat still for half an hour, reading the question over and over, trying to figure out what interference had to do with anything in the question.
I really just need to know, where to start. I have no idea how to begin to attempt this question

 

[rude man]

Do you think the 350 nm specification might have anything to do with it? Could there be reflections from more than one surface?

 

[necromanzer52]

Well, this is just the first part of the question.

Oh yeah, reflections from more than one surface. That's a lot of help actually. Thanks.
I'd ask for more hints, but I have to go hand it in now.