if i have a problem...
would the substitution be..
I know it would be for = xe^-2x..
It's not clear what you are asking. You seem to be conflating "variation of parameters" and "undetermined coefficients".
Assuming you mean , which has and as independent solutions to the associated homogenous equation, in order to use "variation of parameters" you would have to use . If you are referring to "undetermined coefficients", you would try .