Consider the following premises. forall x_11: R(x_11) <-> S_11(x_11). ~R(a_11). ~S_11(a_11) -> u. Can we infer u from them? If possible, name the inference rules being used at each step.
 From forall x_11: R(x_11) <-> S_11(x_11) we can infer R(a_11) <-> S_11(a_11) via universal instantiation. From R(a_11) <-> S_11(a_11), ~R(a_11) we can infer ~S_11(a_11) via biconditional elimination. Finally, from ~S_11(a_11) -> u, ~S_11(a_11) we can infer u via modus ponens. Therefore, the answer is yes.