Consider the following premises. u or ~p_2. u -> s_1. s_1 -> p_2. Can we infer s_1 -> u from them? If possible, name the inference rules being used at each step.
 From u or ~p_2 we can infer p_2 -> u via rewriting. Finally, from s_1 -> p_2, p_2 -> u we can infer s_1 -> u via transitivity. Therefore, the answer is yes.