Consider the following premises. u or ~s_2. u -> s_4. s_4 -> r_6. Can we infer ~(s_2 -> r_6) from them?
 From u -> s_4, s_4 -> r_6 we can infer u -> r_6. From u or ~s_2 we can infer s_2 -> u. Finally, from s_2 -> u, u -> r_6 we can infer s_2 -> r_6, which contradicts ~(s_2 -> r_6). Therefore, the answer is no.