Consider the following premises. If Elisabeth were a sailor, then Sarah is a soldier. If Sarah were fast, then Elisabeth is a sailor. For all x_4, x_4 is fast if and only if x_4 is a soldier. Can we infer the following from them? If we can, name the inference rule being used: Sarah is fast if and only if Sarah is a soldier.
We can infer this via the universal instantiation rule. Therefore, the answer is yes.