Consider the following premises. u <-> s. w_2 <-> u. ~w_2. Can we infer ~s from them? If possible, name the inference rules being used at each step.
 From w_2 <-> u, ~w_2 we can infer ~u via biconditional elimination. Finally, from u <-> s, ~u we can infer ~s via biconditional elimination. Therefore, the answer is yes.