In using-lax-milgram-to-find-a-weak-solution-in-an-intersection-of-sobolev-spaces the weak solution for $$ -\Delta^2 u = f \in L^2(U)\\ \\ u|_{\partial U}=\Delta u|_{\partial U} = 0 $$ was discussed, I have a question about the week solution of $$ \Delta^2 u + u = f \in L^2(U)\\ \\ u|_{\partial U}=\Delta u|_{\partial U} = 0 $$ I think I should use coupled elliptic PDE theory, Any hint or suggestion is helpful for me.

In advanced thanks from anyone who tries to help me. I also asked this question in and got solution by dear @Tomas (thanks to him) link