How Is Work Done by Friction Calculated for a Crate Pulled at a Constant Speed?

[JSmith2009]

Homework Statement

A rope that is 30° to the horizontal has a tension of 150 N. It is used to pull a 40kg packing crate on a rough sruface through a distance of 6 m. If the crate moves at a constant speed, find the work done by friction.

Homework Equations

Energy formulas, not kinematic.

The Attempt at a Solution

I'm at a loss for what step to take after drawing my F-B-D. I've got it set up, but just brain-farting what next. I've thought over this problem, and I know it's a very simple one, but I just can't comprehend it or something.

 

[heth]

"constant speed" is question-code for something...