Consider the following premises. t_2 -> u. ~u. w_3 <-> t_2. u_3 <-> w_3. Can we infer ~u_3 from them? If possible, name the inference rules being used at each step.
 From t_2 -> u, ~u we can infer ~t_2 via modus tollens. From w_3 <-> t_2, ~t_2 we can infer ~w_3 via biconditional elimination. Finally, from u_3 <-> w_3, ~w_3 we can infer ~u_3 via biconditional elimination. Therefore, the answer is yes.