Answer
U = 13.0 J
Work Step by Step
From problem 81h, we got that U = 13.0 J at x = 12.0 m. From x = 12.0 m to x = 13.0 m, there are no forces acting on the particle. Therefore, no work is done on the particle and the potential energy at x = 13.0 m remains the same, U = 13.0 J.