Consider the following premises. ~u_4. s <-> q_6. q_6 -> u_4. u_4 -> q_6. Can we infer ~s from them?
 From u_4 -> q_6, q_6 -> u_4 we can infer u_4 <-> q_6. From u_4 <-> q_6, ~u_4 we can infer ~q_6. Finally, from s <-> q_6, ~q_6 we can infer ~s. Therefore, the answer is yes.