Answer:
The car was 12.8m/s fast when the driver applied the brakes.
Explanation:
The equations of motion of the car in the horizontal and vertical axes are:

Since the kinetic friction is defined as
and
we have:

Next, from the kinematics equation of speed in terms of distance, we have:

Since the car came to a stop, the final velocity
is zero, and we get:

Finally, plugging in the known values, we obtain:

It means that the car was 12.8m/s fast when the driver applied the brakes.