The differential equation system is
To solve this differential equation requires a particular solution xp(t) and two fundamental solutions of the homogeneous equation corresponding X1(t) and X2(t)
I find X1(t) and X2(t) for the following cases
Overdamped:
Underdamped
r -> imaginary part
How do I find the particular solution xp(t) ?
You have found the homogenous solution. Now you have to add to it the particular solution. You have already been told how to find the particular solution. All this stuff will be found in any decent textbook that covers second order differential equations with constant coefficients (go to your school library) and I'm certain that Google will turn up pages and pages of links.
Your problem is attempting abstract application questions like these before developing a sufficient grasp of the basic theory in a sufficient number and variety of concrete cases. You are advised to go back and review that theory.