I have d^2y/dx^2 + dy/dx - 4y = x^2

Setting it equal to 0 I have lambda(l) = (-1+17^1/2)/2 or (-1-17^1/2)/2 so yCF(x) = Ae^l1x+Be^l2x

Could someone tell me how to find the particular integral, I have an example where you set yPI(x) = cx+d but this doesn't seem to work and I'm not sure where it comes from, thanks