$\displaystyle m\frac {d^2x}{dt^2} + 5kx = -mgsin\alpha + 11kl_0

$

I am trying to find the general solution.

Setting $\displaystyle \omega^2=\frac{k}{m}$, I found $\displaystyle x_c$ to be:

$\displaystyle x(t) = A~cos \left ( \sqrt 5 \omega t + \phi \right )

$

I have no idea how to find $\displaystyle x_p$...

What trial solution would you suggest using?