Consider the following premises. u_3 or ~q. u <-> s_2. s_2 -> q. t -> u. Can we infer ~(t -> u_3) from them?
 From u <-> s_2 we can infer u -> s_2. From u_3 or ~q we can infer q -> u_3. From u -> s_2, s_2 -> q we can infer u -> q. From u -> q, q -> u_3 we can infer u -> u_3. Finally, from t -> u, u -> u_3 we can infer t -> u_3, which contradicts ~(t -> u_3). Therefore, the answer is no.