Answer
U = 11.0 J
Work Step by Step
From Problem 81c, we got that at x = 0, K = 3.00 J. Since only conservative forces have acted on the particle, the mechanical energy $E_{mec}$ is conserved. Initially, $E_{mec}$ = K + U = 14.0 J + 0 = 14.0 J. So, since at x = 0, K = 3.00 J, we must have U = 11.0 J, so that we still get $E_{mec}$ = K + U = 14.0 J
