y = -dy_2
saying the y value is related to -1 times the second derivative.
Suppose y, y1, and y2 are 3 points separated by a certain delta x. Then we can solve for y in terms of y1 and y2 like so:
Here I've substituted an expression for the slope of change in slopes as you go from y2 to y1 and then to y, and placed that instead of -dy_2 in the original differential equation.
Then if you know two starting points, like y1=0.8572989892, y2=0.8521080219 for a delta x of 1/100. Then you can generate a new y value, then make y2=y1, y1=y and repeat generating the graph....