You mean you have found as a particular solution to .

You're expected to know that the solution to is the homogeneous solution plus a 'total' particular solution.

The homogeneous solution is the solution to and you're expected to be able to solve this.

You have a particular corresponding to the term on the right hand side of the DE. Now you need a particular solution corresponding to the term . I suggest trying one of the form and getting a and b in terms of y'(0) and y(0). Then the 'total' particular solution is .

If you're stuck on finding the homogeneous solution or the 'total' particular solution, post what you've done and where you get stuck.