Consider the following premises. ~r_1. s_3 -> r_1. u -> s_3. Can we infer u from them? If possible, name the inference rules being used at each step.
 From s_3 -> r_1, ~r_1 we can infer ~s_3 via modus tollens. Finally, from u -> s_3, ~s_3 we can infer ~u via modus tollens, which contradicts u. Therefore, the answer is no.