Calculate coordinates of edges of given picture

Hi there,

I just can’t find it out by myself for days now so I’d really appreciate any concrete formulas as an answer
to my following question:

How do I calculate the coordinates of the edges of the bitmaps returned by the following link
a) in WGS-84
b) in Mercator?,48.7123&zoom=15&type=mapnik&width=700&height=500,48.7123&zoom=14&type=mapnik&width=700&height=500

The two links only differ in the zoom level.

I’d be really happy if you write down the formulas for it instead of given me any links to wiki-articles
since I did read them, however did not understand as much to be able to use it.

Thank you very very much in advance,

Bye, bye Roland