Consider the following premises. r_6 -> p. p -> u_3. u_3 -> w. w -> r_6. Can we infer u_3 <-> p from them? If possible, name the inference rules being used at each step.
 From u_3 -> w, w -> r_6 we can infer u_3 -> r_6 via transitivity. From u_3 -> r_6, r_6 -> p we can infer u_3 -> p via transitivity. Finally, from u_3 -> p, p -> u_3 we can infer u_3 <-> p via biconditional introduction. Therefore, the answer is yes.