I think you have got the right answer. But I think the way you calculated it and your reasoning are not right. You've used the force on the particle at 5 meters, and multiplied by 5 meters, but what would that actually mean? That the particle was being acted on by a constant force over 5 meters? The actual situation is that the particle is acted on by a varying force from infinity to the 5 meter point. I think it is just coincidence that you got the right answer. There's two possible ways to answer this question. One involves using the potential, and the other way involves integrating the force over the path. Are you familiar with these methods?