At an accident sense on a level road investigators measure a car's skid mark to be 88 m long . The accident occurred on a rainy day , and the cofficient of kinenatic friction was estimated to be 0.42 . Use these data to determine the speed of the car when the driver slammed on (and locked)the brackes ( Why does the car mass doesnot matter?)

The braking force is - \mu m g, and using F = ma you get


F = ma = -\mu mg


So you can solve for 'a,' and from that figure out the initial velocity. Note how the 'm' terms cancel out - the idea being that while it takes more force to slow a heavier object, the friction against the road (and hence stopping force) is greater for a heavier object.