Consider the following premises. u_4 <-> p_1. p_1 or s_1. u_4 -> ~u. u. Can we infer s_1 from them?
 From u_4 -> ~u, u we can infer ~u_4. From u_4 <-> p_1, ~u_4 we can infer ~p_1. Finally, from p_1 or s_1, ~p_1 we can infer s_1. Therefore, the answer is yes.