Answer
The kinetic energy of the body will be $8.0~J$ at $~~x = 4.0~m$
Work Step by Step
In part (a), we found that the kinetic energy at $x = 3.0~m$ is $12.0~J$
From $x = 3.0~m$ to $x = 4.0~m$, the work done by the force is $-4.0~J$
Note that $~~K_f = K_i+W$
Therefore, the kinetic energy of the body will be $8.0~J$ at $~~x = 4.0~m$.
