Center-of-mass work stopping distance

[epicbattle]

Homework Statement

To avoid an accident, the driver of a 1000kg car moving 90km/h on a straight horizontal road steps o the brakes with maximum force. The car skids to a stop. The coefficient of kinetic friction between tires and road is 0.80. How far does the car travel during the skid?

Homework Equations

Integral of F(net)*dL from 1 to 2 = delta K (transfered)

This is actually solved in my book, but I don't understand how they set it up.
They set it up like:
F(net) = F(n) +mg + F(k) = F(k)

where n is normal force and F(k) is force of kinetic friction.

Why do they set it up so the forces are equal to F(k)?

It is unnecessary to solve. Just an explanation for the setup is what I am looking for. Thank you very much.

 

[Doc Al]

F(net) = F(n) +mg + F(k) = F(k)

I assume that this is a vector equation. F(n) acts upward and is equal and opposite to mg which acts downward. Those two forces cancel, leaving only the friction force.

 

[epicbattle]

...I feel silly. Thank you very much.