Consider the following premises. For all x_3, if x_3 were a janitor, then x_3 is playing tennis. For all x_3, x_3 will play tennis if and only if x_3 is a janitor. Can we infer the following from them? If we can, name the inference rule being used: For all x_3, x_3 is a janitor if and only if x_3 will play tennis.
 From the fact that for all x_3, x_3 will play tennis if and only if x_3 is a janitor we can infer that for all x_3, if x_3 plays tennis, then x_3 is a janitor via universal biconditional elimination. Finally, from the fact that for all x_3, if x_3 were a janitor, then x_3 is playing tennis, and that for all x_3, if x_3 plays tennis, then x_3 is a janitor we can infer that for all x_3, x_3 is a janitor if and only if x_3 will play tennis via universal biconditional introduction. Therefore, the answer is yes.