Consider the following premises. For all x_5, if x_5 drives a car, then x_5 is a janitor. If William were a janitor, then William is driving a car. Can we infer the following from them? If we can, name the inference rule being used: If is not true that William is a janitor if and only if William is driving a car.
 From the fact that for all x_5, if x_5 drives a car, then x_5 is a janitor we can infer that if William drives a car, then William is a janitor via universal instantiation. Finally, from the fact that if William were a janitor, then William is driving a car, and that if William drives a car, then William is a janitor we can infer that William is a janitor if and only if William is driving a car via biconditional introduction, which contradicts that If is not true that William is a janitor if and only if William is driving a car. Therefore, the answer is no.