What is the formula for calculating the speed of a car which leaves a skid mark of 35metres?

Calculating this depends on a very large number of factors. Ranging from the make of the brakes to the type of road that you are driving on. Even then formulas usually only give an estimated speed as brakes don't lock on instantly.

More information can be found at:
http://www.harristechnical.com/articles/skidmarks.pdf

This website will explain the different variables involved.