Once we isolate the radical, our strategy will be to raise both sides of the equation to the power of the index. If the length of the skid marks is d feet, then the speed, s, of the car before...Once we isolate the radical, our strategy will be to raise both sides of the equation to the power of the index. If the length of the skid marks is d feet, then the speed, s, of the car before the brakes were applied can be found by using the formula If the length of the skid marks is d feet, then the speed, s, of the car before the brakes were applied can be found by using the formula s=√24d.