Consider the following premises. t_2. u <-> t. t_2 -> t. Can we infer t from them? If possible, name the inference rules being used at each step.
 From u <-> t we can infer u -> t via biconditional elimination. From t_2 we can infer t_2 or u via addition. Finally, from t_2 -> t, u -> t, t_2 or u we can infer t via disjunction elimination. Therefore, the answer is yes.