Consider the following premises. ~s. t_2 -> u_3. s <-> t. t or t_2. Can we infer u_3 from them? If possible, name the inference rules being used at each step.
 From s <-> t, ~s we can infer ~t via biconditional elimination. From t or t_2, ~t we can infer t_2 via disjunctive syllogism. Finally, from t_2 -> u_3, t_2 we can infer u_3 via modus ponens. Therefore, the answer is yes.