Consider the following premises. t_2 -> u_2. s <-> u_2. t_2 <-> s. Can we infer t_2 -> s from them?
 From s <-> u_2 we can infer u_2 -> s. Finally, from t_2 -> u_2, u_2 -> s we can infer t_2 -> s. Therefore, the answer is yes.