Consider the following premises. s <-> u_2. q_4 <-> u_2. ~q_4. Can we infer ~s from them? If possible, name the inference rules being used at each step.
 From q_4 <-> u_2, ~q_4 we can infer ~u_2 via biconditional elimination. Finally, from s <-> u_2, ~u_2 we can infer ~s via biconditional elimination. Therefore, the answer is yes.