How much work does F do in moving the cart 3.0 m?

Assuming the coefficient of kinetic friction is 0.60...

Draw a free body diagram listing all forces (I can't really show that to you on here). Since the speed is constant, the acceleration must be zero and thus the net force must be zero, so the horizontal force F must be equal to the frictional force.

Frictional force will be equal to the normal force (equal to mg in this case), multiplied by the coefficient of kinetic friction:

F = Frictional force = umg

where u is the coefficient of kinetic friction, 0.6, m and g are mass and gravity.

F = 0.6*20*9.8

Now just use

Work = F*displacement

where displacement is 3 m.