(3-6A)

The work-energy theorem says that the work done on an object is  equal to the change in the object's kinetic energy. 

See if you can confirm the truth of this, in the case where the applied force is constant. Use as a starting point, this classical equation of motion:

v2 = u2 + 2aΔx      

(where u and v are the initial final velocities; a is acceleration and Δx is distance moved)

together with Newton's second law (F = ma).