Answer:
The coefficient of kinetic friction

Step-by-step explanation:
From Newton's second law

If the velocity is constant, that means the summation of all forces must be equal to zero. Draw the free-body diagram to obtain the sums of forces in x and y. It must include the Friction Force, in the opposite direction of the displacement, the weight (
), the Normal Force, which is the is the consequence of Newton's third law and the forces from the two workers.
The sum in y is:

Solving for the
:

The sum in x is:

Solving for the
:

The formula of the magnitude of the Friction force is

That means the coefficient of friction is:
