A horizontal force of 6 N acts on the body. The body does not accelerate but there is a displacement of 5 m. (Assumed to be in the horizontal direction).
The work done on an object by a force is the dot product of force and displacement. If the body does not accelerates, the force applied is being counter by a force of equal magnitude in the opposite direction. Usually this is the frictional force.
Here, the work done by the 6 N force is 6*5 = 30 J.
There is no acceleration of the body as there is a force of 6 N in acting in the opposite direction.
The displacement of 5 m due to the force indicates a work done by the 6N force though the resistive force does an equal amount of work that makes the net force 0.