question of mathematics (involving physical as well)?

The skid marks made by a car said his brakes were applied fully for a distance of 75 meters before coming to a stop. the car in question is known to have a constant deceleration of 20 m / s ^ 2 in these conditions, speed in km / h was the car traveling when the brakes are first applied? I know constant acceleration means you can use x (t) = 1/2a (t) ^ 2 + v (T) + x but I can not seem to calculate the time. and then use the time to solve for initial velocity. I tried to make 75m/20m/s for 3.75 seconds for the car to stop but I went with a small initial velocity, too. (17.5 million / s) Any help?

v = u + a * t ————– 1 (V-final speed -u initial velocity, acceleration, t-time) where, v = 0, so t = – (u / a) s = u * t + 1 / 2 * t * a * t —- 2 where s = 75 m and 20 m / (sec * sec) Equation 2 becomes s =- = u * (-u / a) + 1 / 2 * a * (-u / a) * (-u / as) = -1 / 2 * u * u / s put a = 75, = -20 u * u = 3000 u = 54.78 m / sec

How It’s Made – Luxury Cars (Rolls Royce Phantom)