Consider the following premises. ~u. t_1 <-> s_2. u <-> t_1. Can we infer ~s_2 from them? If possible, name the inference rules being used at each step.
 From u <-> t_1, ~u we can infer ~t_1 via biconditional elimination. Finally, from t_1 <-> s_2, ~t_1 we can infer ~s_2 via biconditional elimination. Therefore, the answer is yes.