Okay, here's a revised version of what I wrote this (Monday) morning... Okay, it's completely rewritten, hiding my shame!
When we do a Free Body Diagram on the box, we find that the magnitude of the normal force on the box is simply equal to the magnitude of the weight. So:
Work done by friction, then, is:
Kinetic friction is always defined to be in the direction opposite the displacement. So and are at 180 degrees to one another, sooooooooo:
You can plug in the numbers from here. And the "-" sign is very important as it implies energy is leaving the system, so you have to include it.