A horizontal force of 80n is required to push a piece of furniture across a floor to the right at a constant velocity. how much friction acts on the furniture when it is at rest on a horizontal surface

0, Net force is zero (because velocity is constant!) Key word: "constant velocity" meaning no acceleration. This means net force has to be zero because if there was a force pushing it faster and faster, it wouldn't have constant velocity and it would be accelerating.