$\displaystyle x^{2}e^{x}-xsin4x+x^{3}$
See this and see if you can figure it out from there.
Another thing to note is the following: If $\displaystyle L_1,L_2,L_3$ are annihilators of $\displaystyle f(x),g(x),h(x)$ respectively, then $\displaystyle L_1L_2L_3$ annihilates $\displaystyle f(x)+g(x)+h(x)$.