Consider the following premises. t_2 <-> s_1. s_1 -> ~u_4. u_4. Can we infer ~t_2 from them? If possible, name the inference rules being used at each step.
 From t_2 <-> s_1 we can infer t_2 -> s_1 via biconditional elimination. From s_1 -> ~u_4, u_4 we can infer ~s_1 via modus tollens. Finally, from t_2 -> s_1, ~s_1 we can infer ~t_2 via modus tollens. Therefore, the answer is yes.