I don't have a clue of what you are trying to do... What is that graph about? What are the axes? What graph is drawn on it?
From what I can gather from your units and general knowledge your force constant is going to be the Force/distance (does this happen to be a question on Hooke's Law by that way?)
Your force constant is going to be the gradient on the graph of Force (y axis) against distance (x axis)