Length needed from a known length and angle

Hi,

I am not sure what the right title should be or even what area of maths this is (geometry?)

It's only been 6 or 7 years since I did this at school... can't believe how easy it is to forget, if you don't do it everyday.

I am currently designing a piece of metal work and need to work out the depth of a screw.

The datasheet says ...

http://i27.tinypic.com/24pw08g.jpg

However I'm completely stumped at what the formula for finding this out is! If someone could show me the formula, that would be great.

Thanks

Ps. apologies for the bad pic ! The screw is equal angles at both sides.