Runge Kutta on second order differential

Hi, As part of my project I have been asked to code a program that peforms the runge kutta 4th order algoritm on a equation of form:

$\displaystyle ax'' + bx' + c = 0 $

The way I have been told to do this is use x_1 and x_2 and use them like this:

$\displaystyle ax'' + bx' + c = ax_1 +bx_2 +c $

and then perform the runge Kutta algoritm on x_1 and x_2 serperately.

I haven't done anything on second order differentials for awhile now, could someone tell me how to set this up? Also is an initial condition needed? I think the solution is something like f(x) =e^(Ax).