Runge Kutta - Second order differential again.

Using $ax'' + bx' + c$ as an example.
Set $x = x_1$ and $x' = x_2$
Then $x_1' = x_2 and x_2' = (c -bx_1')/a$

From my previous thread I gathered that I needed to run the runge Kutta algorithm on both of them.

BUT how would I combine them together to form the actual solution?

If you could this step by step what my program needs to do that would be awesome (Evilgrin)