Consider the following premises. p_4 <-> p_9. p_9 <-> u. Can we infer u -> p_4 from them? If possible, name the inference rules being used at each step.
 From p_9 <-> u we can infer u -> p_9 via biconditional elimination. From p_4 <-> p_9 we can infer p_9 -> p_4 via biconditional elimination. Finally, from u -> p_9, p_9 -> p_4 we can infer u -> p_4 via transitivity. Therefore, the answer is yes.