Consider the following premises. t_3 or ~u. t_3 <-> r_4. t_3 -> u. Can we infer r_4 -> u from them? If possible, name the inference rules being used at each step.
 From t_3 or ~u we can infer u -> t_3 via rewriting. From t_3 <-> r_4 we can infer r_4 -> t_3 via biconditional elimination. From t_3 -> u, u -> t_3 we can infer t_3 <-> u via biconditional introduction. From t_3 <-> u we can infer t_3 -> u via biconditional elimination. Finally, from r_4 -> t_3, t_3 -> u we can infer r_4 -> u via transitivity. Therefore, the answer is yes.