Thanks Lnewqban !, is the problem 13.68 form Beer and Jhonston. I misunderstood the question of the problem, the problem ask for additional compress, I got the complete deformation, there is not repeated term Thanks again.
In the initial position the spring is previously compressed, then the block adds a force, and the spring is again deformed. I think the energy balance is incorrect; the potential energy of the spring is repeated.