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