Consider the following premises. ~r_4. t -> r. u_3 <-> r_4. r -> t. r -> u_3. Can we infer ~t from them?
 From t -> r, r -> t we can infer t <-> r. From u_3 <-> r_4, ~r_4 we can infer ~u_3. From r -> u_3, ~u_3 we can infer ~r. Finally, from t <-> r, ~r we can infer ~t. Therefore, the answer is yes.