i have to solve Xn+1 -4Xn +3Xn-1= 36n^2

I did the first part, where i find Xn=A3^n+ B

But when i try to find the solution of the form Xn= Pn^2+ Qn+ R

it turned out weird and i cannot find the P Q R,can anyone help me?

If possible, can someone post a full answer for me? I really need an example for this kind of question.