Consider the following premises. ~t_2. u_3 -> t_2. u_3 or ~t_2. Can we infer u_3 from them? If possible, name the inference rules being used at each step.
 From u_3 or ~t_2 we can infer t_2 -> u_3 via rewriting. From t_2 -> u_3, u_3 -> t_2 we can infer t_2 <-> u_3 via biconditional introduction. Finally, from t_2 <-> u_3, ~t_2 we can infer ~u_3 via biconditional elimination, which contradicts u_3. Therefore, the answer is no.