How would you go about solving this Work-kinetic energy thorem?

[Merlan114]

A 2.1x10^3kg car starts from rest at the top of a driveway that is sloped at an angle of 20 degrees with the horizontal. An average friction force of 4.0x10^3N impedes the car's motion so that the car's speed at the bottom of the driveway is 3.8m/s. What is the length of the driveway?
What i did was:
.5mv^2=gmD(cos20)
.5(2.1x10^3)(3.8)^2=4.0x10^3d(cos 20) and got d= 7.1
I check the back of y book and the answer is 5.1. What did I do wrong? Did I use the wrong formula? How do I go about solving this problem?

 

[rock.freak667]

That frictional force is already along the slope, so the total energy mgh will be equal to the work done by the frictional force + ke at the bottom of the incline.