how would i find the speed of a vehical that put down 42feet of 4 wheel skids prior to impact with another vehical that was stopped.?road serfice is concreat.

Look up the coefficient of kinetic friction for tires skidding on concrete. Call it Uk

M V^2/2 = M g Uk X

M's cancel; solve for V.
g = 32.2 ft/s^2

V = sqrt(2*g*Uk*X)
The number of tires does not matter.

The answer will be in ft/s

For Uk,
http://ffden-2.phys.uaf.edu/211_fall2002.web.dir/ben_townsend/staticandkineticfriction.htm
suggests a value of 0.8