Consider the following premises. t_1 -> s_1. t_1 <-> u. ~s_1. Can we infer ~u from them? If possible, name the inference rules being used at each step.
 From t_1 -> s_1, ~s_1 we can infer ~t_1 via modus tollens. Finally, from t_1 <-> u, ~t_1 we can infer ~u via biconditional elimination. Therefore, the answer is yes.