Consider the following premises. p_11 or ~u_4. ~p_11. Can we infer u_4 from them? If possible, name the inference rules being used at each step.
 From p_11 or ~u_4 we can infer u_4 -> p_11 via rewriting. Finally, from u_4 -> p_11, ~p_11 we can infer ~u_4 via modus tollens, which contradicts u_4. Therefore, the answer is no.