Consider the following premises. w_3 -> s. u. w_3. Can we infer ~s from them? If possible, name the inference rules being used at each step.
 From w_3, u we can infer w_3 and u via conjunction. From w_3 and u we can infer w_3 via simplification. Finally, from w_3 -> s, w_3 we can infer s via modus ponens, which contradicts ~s. Therefore, the answer is no.