Consider the following premises. p_6 or ~u. u_1 -> r_6. p_6 -> u_1. Can we infer ~(u -> r_6) from them?
 From p_6 -> u_1, u_1 -> r_6 we can infer p_6 -> r_6. From p_6 or ~u we can infer u -> p_6. Finally, from u -> p_6, p_6 -> r_6 we can infer u -> r_6, which contradicts ~(u -> r_6). Therefore, the answer is no.