A1500 kg car skids to a halt on a wet road where μk = 0.51. how fast was the car traveling if it leaves 66-m-long skid marks?

We got coefficient of friction uk = 0.51
Mass m = 1500 kg 
normal force on the car = weight of the car = 1500 * 9.8 N = 14700 N 
and the force of friction on the car, f = uk * normal force = 0.51 * 14700 N = 7497 N 
taking Acceleration a = -f/m = -7497/1500 = -4.998 m/s^2 
Displacement s = 66 m 
Final velocity v = 0 
Initial velocity u = ? 
v^2 = u^2 + 2as 
0 = u^2 - 2 * 4.998 * 66
u = 25.69 m/s 

c.the charges if the proton and electron and gravity

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