Question: Runge-Kutta 4th order

Hi, I am working on an assignment and have some doubts about my solution to plotting the position of an anharmonic oscilliator with the Runge-Kutta method.

My attempt on the problem:

Handin_2.mw

A picture of the problem is attached below. In addition to this, we are to repeat the problem with k = 0.1 (f = 0 still), and k = 0.1 and f = 0.5.