Answer
The speed of the car when the driver slammed on the brakes was 21 m/s. The car's mass does not matter because the mass m cancels out of the equation.
Work Step by Step
The work done by the force of friction on the car is equal to the kinetic energy of the car.
$KE = Work$
$KE = Fd$
$\frac{1}{2}mv^2 = Fd$
$\frac{1}{2}mv^2 = mg\mu d$
$v = \sqrt{2g\mu d}$
$v = \sqrt{(2)(9.8 ~m/s^2)(0.30)(78 ~m)}$
$v = 21 ~m/s$
The speed of the car when the driver slammed on the brakes was 21 m/s. The car's mass does not matter because the mass m cancels out of the equation.
