Thanks everyone for your help.
The velocity is constant, and that means that the acceleration is zero. That has something to do with the net force...
OK, but that step was unnecessary. (You were given mg, and that's what you need.)
mk, usually symbolized as ##\mu_k##, is what you are solving for. Use NTW's hint to determine for the force of friction, Ff. Then you can solve for the coefficient of friction.
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